Angel: Angel has one, a heroic sounding anthem, later phased out in the third season. Brought back intentionally for a moment in Season 5 when Angel is in desperate need to rediscover his original mission statement of helping the helpless.
In The Mighty Boosh, whenever a shaman (mainly Naboo) turns his back on someone their leitmotif plays.
On Clarissa Explains It All, whenever Clarissa's friend Sam put his ladder up to her window, he would be introduced by a drawn-out guitar "twang."
The Torchwood Theme, used in series 2, and as the theme tune for Torchwood. Later crops up when Captain Jack comes back in series 3.
Then there's the Master theme, consisting of a four-note drumbeat. He also has a three-note motif the composer admits musically states "The Ma-ster!" for moments of notable eeeevil.
The Delgado Master has his own evil theme music of creepy awesome. It gets repeated so often it becomes a fanfare announcing his arrival before you even see him on screen.
The Eleventh Doctor has a Mike Oldfieldian action theme, "I Am The Doctor", that's something of a spiritual successor to "All the Strange, Strange Creatures". Series Five also introduced motifs for Amy Pond, the new Daleks, the cracks (in the form of a creepy buzz) and a joint theme for Amy and the Doctor.
Bear McCreary, the composer of the re-imagined Battlestar Galactica, has developed leitmotifs for nearly every primary and secondary character on the show, as well as numerous locations, themes, and relationships. He runs a very detailed blog of the process which can be found here. The themes include:
Number Six: this theme appears in virtually every scene where Virtual Six appears to Gaius Baltar, and is played on the gamelan.
The Adamas: their Celtic-inspired theme is often played on a uilleann pipe. Commonly known as 'Wander My Friends.'
Roslin: the theme that most commonly represents her by Season 4 started out as the Destiny (religious) theme, heard prominently in the season 1 finale.
Roslin And Adama: a Celtic-inspired waltz that was first introduced in the "Resurrection Ship" two-parter in season 2 and returned for many of their most intimate moments, it was featured most heavily in the back half of season 4. McCreary called it the only true love theme of the show.
Baltar: a number of short phrases. In the season 4 premiere he gains a new religious theme for his scenes among the cultists.
Lee: due to being so inflexible, it is rarely heard on the show.
Kara: associated with a number of themes, including Starbuck's Destiny (originally used to represent her relationship with Leoben) and the Forgiven theme from season 1.
Lee and Kara: a string-heavy theme inspired by the Shape of Things to Come (Opera House) theme, it first appears over flashbacks to New Caprica in season 3, as well as Lee's attempt to help Kara in 'Maelstrom' and their good-bye in 'Six of One' among other episodes.
Kara and Anders: another string-heavy theme that plays over the two scenes where Kara says good-bye to Sam. In The Plan TV movie it is used for Anders alone.
Tigh: a military-inspired brass theme that first plays in the season 2 episode 'Fragged.' The instruments used vary wildly from season to season, underscoring Tigh's growth.
Gaeta: introduced midway through season 4 when Gaeta is injured, the theme comes in both instrumental and choral variants (sung by Alessandro Julianni, Gaeta's actor, no less). In season 4.5 it sees heavy use during the mutiny arc.
Military theme: used most often for scenes between Adama and Tigh, as well as during the season 4.5 mutiny arc.
Shape of Things to Come: AKA Opera House theme. Plays over the season 1 finale, as well as various destiny-themed moments throughout the seasons.
All Along the Watchtower: chiefly played on the electric sitar and electric violin. This theme first appears in the season 3 premiere, and in 'Crossroads' comes to represent the Final Five, which becomes its primary use in season 4.
Basestar: a piano piece first introduced in season 3 to accompany Gaius Baltar's scenes on the Cylon basestars. Used in season 4 during the Cylon Civil War arc.
Boomer/Athena/Hera: A gamelan theme that's probably among the most commonly heard on the show.
Religious ceremony: the "Two Funeral" theme from season 1, also the US season 1 theme, is heard at a lot of funerals.
Every Power RangersBig Bad (and at least one Dragon) is recognizable by music, with Lord Zedd's being the most well-known (Rita even used it instead of her own when she appeared in front of Rito and Goldar in Power Rangers Zeo episode "Mondo's Last Stand"). A few rare good guys have a song (early in the series) or their own little tune, and there was one instance of a mystery Ranger's identity having a nicely sneaky Reveal: He mentioned that they'd met him before, at which point the intro to the music that always accompanied a certain little boy from an earlier episode is played. Sure enough, it turns out the mystery Ranger is that child's adult self, sent back in time.
Power RangersReunion Shows will bring back the theme song for each series that returns. The 15th anniversary episode "Once A Ranger" (part of Power Rangers Operation Overdrive) makes it a little absurd, however: Rangers from five teams (Mighty Morphin, Ninja Storm, Dino Thunder, SPD, and Mystic Force) appear and we get to hear all five of their respective season theme songsnote Except Mighty Morphin's, due to legal issues - it's believed that the producers wouldn't have spent money for the right to use it after all they spent bringing the actors in each of the three battles in which they participate. The big final battle throws in the Operation Overdrive theme as well, since the battle is the Operation Overdrive team's collective He's Back moment following their earlier De Powering.
Although, the most recognizable music from Power Rangers after the main theme has to be Bulk and Skull's theme, which is kind of silly and complimented with gag noises. Possibly due to the fact that the pair remain the longest-running recurring characters of the series.
Lightspeed Rescue team had a notable motif drawn from the guitar solo/bridge from its theme song.
Star Trek has several memorable motifs: The Enterprise fanfare, the Klingon theme, the fight music of doom from the Original Series...
The fight music was used in The Cable Guy in a clever Shout-Out to the Spock vs. Kirk fight: Jim Carrey's character actually sings along to the music while it plays in the background. It also highlights just how unbalanced he is, as he takes the fight as seriously as Spock does and almost winds up killing Matt Broderick several times.
Also used on Futurama, in an episode more or less directly parodying the Original Series' Amok Time - when Fry hears the iconic fight music, he knows he's really in trouble.
In that episode it happened to be the national anthem of the planet they were on.
Composers seem to love putting their own personal stamp on Mr. Spock, who has had three different leitmotifs to his name, all of which are very different from each other, though they all have a certain wistfulness to them...
His TOS leitmotif, by Gerald Fried. This is the one with the lone bass guitar, though it was often played by the string section as well.
The premiere episode of Star Trek: Deep Space Nine, "Emissary" had a very effective use of mixed motif to evoke Passing the Torch. List when the Enterprise undocks to leave: a bar of the TNG theme, usually used to end an episode leads directly into a bar of the DS9 theme.
In Smallville, everytime some dramatic something hints at Clark's future as Superman, a few bars of the theme music from Superman: The Motion Picture can be heard.
The syndicated series The Adventures of Superboy had themes for just about every character and many other elements of the show, including: Lex Luthor, Bizarro, Metallo, Smallville, kryptonite, and the Bureau For Extra-Normal Matters (Clark's workplace in season 3 and 4). Even minor villains that appeared only in one or two episodes got their own themes.
Bill Thompson in LazyTown has a hard rock guitar riff when he confronts the "evil dude" by running into him, entering his liar, etc.
Bridget, from 8 Simple Rules hears a short guitar chord every time the cool new guy, Damian appears. She is the only character to notice the music, which stops when she falls for a nerd instead.
A little Lampshade Hanging went on as well, since Bridget would ask if anyone else could hear the guitar.
Claude's theme involves wind and voices to create the feeling of a ghost-like presence in reference to his invisibility.
Sylar's involves marimbas and bassoons with staccato to re-create the sound of clocks ticking in reference to the character's ability to see how things work. Subverted when Sylar gains shapeshifting powers. You see Mr. Bennet walking down the hallway while Sylar's theme plays; he looks incredibly smug, which he really shouldn't be, given that Sylar has successfully used his new ability to completely discredit Bennet and nearly destroy his marriage. So, obviously, you conclude that this must be Sylar shapeshifted into Bennet, right? Wrong! Makes the Batman Gambit actually working that much more believable, if you- with all your Genre Savvy- have just fallen for it from beyond the Fourth Wall.
Matt Parkman's theme involves voices being played backwards when he uses his power of telepathy.
Peter Petrelli's theme involves marcato strings.
Niki Sanders' theme was based on her character's alter-ego named Jessica and involved winds and Indian voices chanting in an underscore to give a feeling that she was possessed.
Mohinder Suresh's theme is the piano composition that plays at the end of some episodes.
Pushing Daisies is crawling with them. Emerson's and Oscar's are the most obvious, while the Ned And Chuck music is sufficiently prevalent to double as the show's Theme Tune. And in the episode [...]... Ned and Chuck's bed scene wasn't accompanied by the usual music, which was the most immediate clue that it was actually a Dream Sequence. Of course, Chuck pulling her skin off to reveal Olive underneath was also a bit of a giveaway.
Lester's Theme in Beakman's World played whenever he gave a wrong answer or was generally super annoying. Which was most of the time he was on screen.
Supernatural has "Dean's Family Dedication Theme", a slightly mournful tune played when that love for his Dad and brother gets a little too obsessive.
There's also a much rockier "Brothers' Theme", played during the lighter brotherly moments and distorted a little when the relationship is being strained.
Death is given "Oh Death" by Jen Titus, which is used in his introductory scene.
The first three seasons of Buffy the Vampire Slayer had a tendency to play Buffy and Angel's love theme every time there was a moment related to their relationship on screen. It's a semi-mournful tune most often played on a piano. Apparently the score for season 2 won awards, though that doesn't stop some people from mocking the Wangst factor every time it appeared. Listen here.
Starting in Season Three, the Buffy/Angel theme changes, and undergoes a slow metamorphosis that parallels their relationship coming apart. During the Mayor's advice for them in "Choices" we can hear several of the different themes played back to back, indicating that this is a pivotal scene, at least for Angel
Played with in The Zeppo. The episode is dedicated to showing how awesome Xander is, whether he (or anyone else) realise it or not. Meanwhile, an intentionally trite episode of Buffy's playing in the background. Mid-episode we cut suddenly to a massively OTT emotional scene with Buffy and Angel with this music being played - it cuts off when Xander appears at the door, hesitantly starting to ask for help, then offer help...then when he walks off, the music and the scene crank up again at full tilt while we cut away to follow Xander, instead.
Jenny Calendar picked up a similar semi-mournful theme underscoring her derailed romance with Giles - unfortunately for her it was on the last episode where she was alive. It did play in later episodes where Giles recalls her, or when he sees her in a Drusilla-induced hallucination.
Similarly Willow/Oz and Willow/Tara both had prominent themes, which are played together at the end of "New Moon Rising" as she decides who she wants.
There's also the dramatic music that plays every time Xander and Cordelia make out, played quite clearly for laughs.
And don't forget the plunky little theme that plays whenever Xander and Willow have an illicit "moment."
Stargate SG-1 has "Sam and Jack's Theme" and the "Goa'uld Theme" among others.
Naturally, the SG-1 theme is reused by composer Joel Goldsmith. However, thanks to the larger budget, he was able to do David Arnold's full Stargate overture (though Ra's theme is swapped out for the Ori theme) for the first time since the original movie.
Used frequently in Arrested Development, but especially with Gob. It's to the point that he, in-character, must consider Europe's "The Final Countdown" to be his own personal theme song.
There's even a specific seven-note stab of music used every time Oscar suggests that he's Buster's father.
Every character on LOST has a theme. Some have as many as eight. Locations and abstract ideas receive motifs as well.
Possibly the most famous is "Locke'd Out Again," the emotional theme of John Locke.
Locke's hunter theme from season 4 also qualifies. It also takes an epic turn for the creepy in season 6 when Locke is the Man in Black
Ben's theme, "Dharmacide," is another common one. The eerie main notes, at the very least, play at least once during every important scene involving Ben.
Not to mention that the emotional part of the song played in Ben's "redeeming" moments is tragic all on it's own.
"Make Your Own Kind of Music" by Mama Cass is always associated with Desmond, because it played during the famous opening of season 2 when he was introduced, and again in another episode involving him. In an example of Fridge Brilliance, the title of the song actually applies to the fact that Desmond, in time travel scenarios, is the only person who can "make his own kind of music," or change future events—a concept that wouldn't be revealed for three years.
However, he does have an official theme that plays in songs like "The Constant" and "Distraught Desmond".
Man in Black's theme also has become a favorite, especially when it is featured prominently in the season 6 episode "Across the Sea".
Kate's main musical motif is Patsy Cline music, which always appears in episodes revolving around her.
Charlie's theme (Called on the soundtrack "Charlie's Temptation") consists of many low cello notes strung together beautifully. An especially emotional version of it plays over the Life and Death Theme while Charlie drowns in the Looking-Glass Station
One of the most instantly recognizeable themes is Sayid's because of its distinct melody and how it's often a violin solo. Not that there's anything wrong with that.
Hollywood and Vines is another major theme which is pretty much guaranteed to play in any season finale (as well as the pilot episode) whenever the cast has gathered together to trek across the island towards some sort of climactic encounter.
The Life and Death theme is an absolutely beautiful theme that plays whenever someone dies. A particularly epic/sad version plays in the final moments of the show as Jack staggers through the bamboo grove and dies.
Juliet is often associated with Petula Clarke's "Downtown" which she plays in three of her centric episodes, but also has her own instrumental theme in the series' score, titled "Ocean's Apart".
In the series starring David Suchet, Hercule Poirot has his woodwinds heavy theme that is also used in the credits.
Human Target, in classic action movie style, uses an heroic orchestral leitmotif for main character Christopher Chance in various forms and tones throughout the episodes.
The Amazing Race uses this for certain teams, like when Harlem Globetrotters Flight Time & Big Easy (Season 15) got the Globetrotter theme "Sweet Georgia Brown" and cowboys Jet & Cord (Season 16) had generic Western movie music.
Kamen Rider Double has "Two in One", the Leitmotif for the protagonists, which has a few different remixes throughout the series. The best of these is probably "Futo Wild Battle" from The Movie, which combines it with "Accel's Dash", the theme of Kamen Rider Accel's Super Mode Trial Form.
Chuck on NBC has Tim Jones composing many different themes, the most commonly used of which is a remix of Cobrastyle by the Teddybears. Different mixes of it have been used to fit action sequences since the pilot, though it has been most often used in the third season.
Thomas the Tank Engine: Every single engine has/had their own separate leitmotif for their adventures. Thomas's was a variation on the main theme, Duck's was very lyrical, Donald and Douglas's theme featured bagpipes, etc.
Skins never usually bothered, but Cook will eternally be associated with "Ace Of Spades" (plays at the start of his first episode, the end of his first episode, and his kid brother playing it in Rock Band 2 during his S4 ep).
Whenever Dennis Duffy shows up on 30 Rock, the soundtrack greets him with harmonica music. Which is weird seeing how he's a Recurring Character and none of the actual regulars seem to have their own musical cues.
Liz Lemon definitely has her own, varied instrumental versions of the "Who's That?" song from the Pilot episode.
In Glee, both times April Rhodes is introduced in her episodes, a version of Heart Of Glass is playing in the background.
Sue also has her own: A marching band drumline plays to announce her appearances.
A slower, sweeter version of Summer Nights plays during Sam and Mercedes interactions.
Kurt and Blaine have their own piano theme that plays during poignant moments in their relationship.
In Kamen Rider OOO, every major character gets one, but more noteworthy is that every combo has a Leitmotif which are sang by the belt. It got lampshaded in the very first episode.
Throughout the entire series, Gilmore Girls had a series of acoustic leitmotifs performed by Sam Phillips at important moments.
The old Richard Greene Robin Hood TV show generally had a 10 note intro for the title. Blackadder turned it into a Leitmotif for Robin Hood in the final episode, Back and Forth.
The Ultra Series has the "Wandaba" scat motif that plays when jets launch or the Science Patrol team charges into battle. Here is one example.
The melancholy Stark theme is the most frequently heard, playing in the pieces "Goodbye Brother," "Winter is Coming," "Jon's Honor, and "King of the North." In season 2, it appears at the beginning of "What Is Dead May Never Die," played during Theon's Face Heel Turn, in which it gradually shifts into the Greyjoy theme instead.
The Lannisters have the "The Rains of Castamere" and its variations. It usually manifests as dark, ominous background music, but Tyrion whistles a faster, more sing-song version of it a few times in Season Two, and it finally appears in full in Blackwater. In season 3, a triumphant version is played after Jaime pulls a feat of wit and sheer balls, and then it's played in-universe in the Red Wedding, right before Robb Stark is slain and the Lannisters win the war against him.
The Baratheons have two themes: the "kingly one," used mostly for Robert, Joffrey, and Stannis, is a bombastic fanfare, fitting for the royal family, first heard in "The King's Arrival" and then reprised in "You Win Or You Die," (where it incorporates elements of "The Rains of Castamere") "The Throne Is Mine," and "Wildfire". A dark reprise of it plays as Joffery dies from poisoning at his wedding. The other one, most frequently used for Robert's biological children, is "Black Of Hair", which is reprised in "Bird Without Feathers" (played when Cersei tells Catelyn about her first son who died as an infant), "Await The King's Justice," and "The Throne Is Mine."
Daenerys has two distinct themes. One, a mysterious-sounding, powerful melody which can be heard in "Fire and Blood", has a distinctly Eastern feel to it. (This makes sense: the Targaryens were originally from Valyria, which lies in Essos.) The other is more triumphant, often mixing with the Game of Thrones main theme, and is reserved for her more glorious moments, such as season 1's "Finale" and "Mother of Dragons". In "Mhysa", it's played with EtherealValyrian Chanting.
The White Walkers also have their own, which can be heard in "North of the Wall" and "The Night's Watch".
Jaqen H'ghar has a quiet, distinct tune which plays whenever he makes an appearance or performs an action affecting the plot. A broader, deeper version plays when Arya, Gendry, and Hot Pie escape in "The Prince of Winterfell".
Melisandre and the Lord of Light's theme can be heard in "Warrior of Light", while Stannis' comes from later in the track, when a grander, more percussive sound kicks in. In season 3, Davos gets his own theme with "The Night is Dark".
Ygritte has a slow, heavy, emotional theme that primarily features a low-pitched fiddle and gradually increases in intensity, either sounding uplifting or sad, depending on the scene. It plays whenever something happens to the romantic plot between her and Jon, such as when they climbed the wall together in season 3's "The Climb" or when Jon eventually burned her body in season 4's "The Children".
In The Incredible Hulk, David Banner and the Hulk each have their own, often incorporated into the background music. David's is The Lonely Man Theme, while the Hulk's is a simple four-note theme. The opening song ("Incredible Hulk Main Title - Version 2") incorporates both, alternating lines of the Lonely Man theme with ever-louder repetition of the Hulk's theme. You can hear it here, starting at about 40 seconds.
Jack McGee gets his own leitmotif, a comical theme used in less-dramatic moments.
In the episode The First, Dell Frye's evil Hulk has his own five-note leitmotif.
In season 4 of Dexter, the Trinity Killer is given "Introducing Trinity" and "Trinity Suite", ominous and haunting themes that illustrate his dark nature as a serial killer. It reappears in season 6 when the cops think they may have found him (in reality, Dexter killed him), but it is instead his son who did the murder.
Root in Person of Interest has a unique theme which opens with a series of clear, single notes which acts as the foundation for the main theme.
Finch shares his leitmotif with the Machine, which is appropriate, given he created it.
Reese has his own leitmotif, which plays whenever he intervenes. It's called Knock, Knock.
Dynasty featured highly melodramatic musical themes for most of its major characters and some settings. The most easily recognized would be the rising and falling strings of Alexis's theme as well as the theme associated with La Mirage. A compilation video can be seen here.