Popeye used to sing his theme song in every cartoon in the really old days, and, as time went on, he still contented himself with humming or scatting the theme of the tune while walking down the street.
Later in the Fleischer series and from then on, the Popeye theme would be used when he ate his spinach.
In The Powerpuff Girls episode "The Powerpuff Girls' Best Rainy Day Adventure Ever," the girls stay inside and pretend to be themselves, humming their own theme song. One of only a swarm of hilarious Mythology Gags and more normal jokes from that particular episode.
In The Simpsons episode "Bart Gets Famous", Bart comes downstairs whistling the Simpsons theme song only to be reprimanded by Marge for whistling "that annoying tune." (This is the first of many self-referential jokes within this episode.)
The producers of The Simpsons were thinking about adding a groan / scream to the beginning of the theme, simply because of the odds that someone would be groaning at hearing it again.
In "Angry Dad - The Movie", the theme song is played when Bart wins an award.
The same thing happened with Homer in "Brother Can You Spare Two Dimes".
In "22 Short Films About Springfield," right before hitting Chief Wiggum with his car Snake can be heard listening to a remix of the show's theme.
One Casper the Friendly Ghost Christmas special features the title ghost meeting up with a girl named Holly Jolly (yes, really). Holly, embarrassed by her, ah, seasonal name, tells Casper to imagine how embarrassing it would be to have "Holly Jolly" for a name — Casper proceeds to imagine his own theme song, with the words "Casper" replaced with "Holly Jolly". Its Ear Worm tendencies are also lampshaded near the end of the scene:
Casper: Oh, great. Now I've got that song stuck in my head. ''(flies off with the "Holly Jolly the Friendly Ghost" song playing in his head}
Kim Possible uses this in several ways. The Kimmunicator's ringtone is a distinctive four-note sequence that's not only in the theme song, but is emblematic of the series. In one episode, the team faces a group of professional heroes for hire with their own theme song, and Ron spends the rest of the episode writing one for himself and Kim. He comes up with, of course, the show's theme song.
Either Kim or Wade must be a fan of Jack Bauer. Listen to the Kimmunicator, then the ring tone in 24, and tell me they're not similar. Go on, do it.
Also in Sitch in Time, she gives Ron his own "Ronmunicator" so he can "Call me or beep me, you know, if you want to reach me." Further, in So the Drama, Kevin's dad says he'll give Kevin her number, so he can "call you, bee—" (and that's as much as we get, but we all knew where it was going).
And another episode featured Beethoven attempting to compose his fifth symphony. Songs he tries and rejects include the themes of Looney Tunes, Tiny Toon Adventures, and Animaniacs. Apparently the writers liked this gag.
The Titans' communicators play a tinny, ringtone-style version of their title theme's primary lick.
The Charlie Brown and Snoopy Show: There's a skit surrounding jack-in-the-boxes, each of which plays the show's theme song. In another skit, Snoopy plays an organ grinder playing the theme tune as well.
Charlie Brown's Christmas Tales has Snoopy playing a little music, only for Lucy to pass by and comment that "O Suzanna" wasn't very Christmas-y. In response, Snoopy switches to "Christmastime is Here", from the original A Charlie Brown Christmas.
At the end of the This is America, Charlie Brown episode on American music, Charlie Brown is asked his favorite song, and he replies that it's one by Vince Guaraldi. "I believe it's called "Linus and Lucy" — by coincidence."
Classic example: The Smurfs are prone to la-la-ing their own theme song.
In an episode of Arthur, while Binky is giving advice to D.W., he begins to recite the lyrics of the the show's theme song. When the familiarity is pointed out, he comments that he heard it somewhere but can't quite remember where.
Another episode has an instrumental of the theme being played off a CD. Arthur comments it sounds familiar, and Buster wonders if the music contains subliminal messages.
Hardhead sings the ending theme at a karaoke party in the Transformers Headmasters episode "The Shadow Emperor, Scorponok".
Kakuryu sings a bit of the end theme to Transformers Victory in an early episode. Considering the theme tune sings about driving villains away, and The Dinoforce eventually turns against Deathsaurus, this qualifies as foreshadowing.
In the pilot episode, when Cheetor encounters Waspinator, he first thinks it's a real wasp. When he realizes it's not, he says it must be "Robots in Disguise!", the famous line from the original Transformers theme.
Animated once showed Sari humming the theme song while listening to some music on headphones, even though the music playing sounded different. In another episode of Animated, Bumblebee left an auto parts store with a car horn that played the first few notes of the theme song. An in this short, Ratchet whistles it.
On A Pup Named Scooby-Doo, at the end of an episode, two pop stars sing their new song, the theme song of the show.
One of the Scooby-Doo movies has Velma singing the classic theme song for a concert.
In the episode "The Once and Future Thing Part 1", when the time-displaced Justice League members saddle up with some of DC's other Old West characters and take off to the villain's hideout, a Western version of the original Justice League theme plays.
Coincidentally, the "Western Justice League" consists of seven members, the same as the show's original League.
Garfield and Friends often had characters humming or softly singing the theme songs. Interestingly, this they'd continue to sing the original theme even after the show had switched to its second theme song; possibly because the new one, while less inappropriately saccharine, was not as easy to hum.
Lanolin even parodies the theme in one episode: "Friends are there to help you get started, but once you get started, who needs 'em?"
The Spanish version of "Fortune Kooky" has Wade singing "Friends are there" in Spanish while walking. In the original version, he was happily singing a generic song.
Orson would eventually sing the second theme song in "The Wolf Who Cried Boy" while bathing. The instrumental version of this music sounds almost like a kazoo playing it.
And Nermal hums the theme song while carving meat she received from Herman Post in the episode "Cute For Loot".
At the end of an early episode of X-Men: Evolution, a relaxed, jazzy tune plays over the kids' enjoying themselves in the pool. It's a version of the early '90s X-Men cartoon theme.
It shows up again during Wan's final battle with Vaatu.
In an episode of Care Bears: Adventures in Care-a-lot, Cheer, Harmony, Trueheart, and Love-a-lot start a band and are heard practicing the show's theme song.
In an episode of Darkwing Duck a brief scene shows Megavolt pacing in his lighthouse tower, singing a version of the title theme, with no lyrics. He soon breaks off and complains, "Agh, I can't get that song out of my head!" His irritation is understandable, since the song is all about his nemesis.
In The Spectacular Spider-Man, Shocker, Ox, and Ricochet are riding an elevator, and Ox starts to hum the show's theme song, prompting the other two to stare at him. "What? It's catchy."
On his way to Hobo 13 in the episode of the same name, Invader Zim shows off to GIR while flying the Voot Crusier. While doing so, he starts sing the theme song.
At the beginning of the Dungeons & Dragons cartoon episode "Child of the Stargazer", Diana is humming the D&DC theme-music.
One episode of Chowder had Shnitzel literally facing the music, which happens to be the show's theme song. Chowder cheerfully hummed along while Shnitzel begged them to stop.
Done in another episode when the tune plays on the radio and Mung Daal sings along.
Undergrads has Good Charlotte guest star as the band preforming at a fair State U is having. Nitz says something along the lines of "Good Charlotte? What have they done that I've heard of?" Someone immediately walks by, listening to the show's theme song on an MP3 player. The band later sings the song at the fair itself.
An episode of Horrid Henry has the titular character sing the theme song to a group of OAPs. He says that it's an "old classic" of "The Killer Boy Rap Stars" (Henry's favourite band).
In Caillou's Holiday Movie, Caillou performed an original Christmas song to the tune of the show's theme song.
One episode of Megas XLR has Coop teaching an S-Force friend how to sing Karaoke. We don't get to hear the music, but we do see that the lyrics are the show's theme tune.
Not a theme tune, but in the VeggieTales episode "Madame Blueberry," the episode's Silly Song ("His Cheeseburger") shows up as muzak playing in the food court.
Songs from earlier episodes, especially the Hairbrush song, actually turn up in the background a lot during different episodes.
Done as a deliberate fourth-wall breaker on Samurai Jack, where a boy whose family Jack stays with overnight expresses his admiration for the famous roving hero by singing a line from the show's theme song.
Several characters have whistled the theme tune for Total Drama Island, most notably Chef in "Paintball Deer Hunt".
And in the third season, it's revealed Chris's ring tone is the theme.
In Wakfu, characters sometimes whistle the opening theme song. Ruel even hums it and references some of the lyrics. A big example of this trope is when Ruel uses a remix of the theme song as the sequence to a dance-pad activated lock to his house right afterwards. Evangelyne also hums it while showering in episode 22.
The end of the Family Guy episode "Believe It or Not, Joe's Walking on Air" had Peter humming the theme song (with the line "Joke's on you" in place of "Laugh and cry" included) when Joe is back to being paraplegic.
In Fish Hooks, many of the characters have the theme song set as their ringtone, most notably Bea.
One episode of Trapdoor had Berk finding a boombox and playing it. What played from it? The full version of the theme tune! The entire episode being nothing more than a glorified music video. At the end of the episode, Berk says he quite liked the song and hoped they'd play it again. Cut to credits and end theme.
Phineas and Ferb has an odd example. While the song doesn't come up, the stuff mentioned/shown in the theme does. They've checked "finding a Dodo" off of a list with several things mentioned in the theme, Buford one time complains that they should have given a monkey a shower (which they actually did early on), and "Painting the Continent" appeared in one of their first episodes ever.
They eventually make up for this in the Big Damn Movie - namely, the song "Summer (Where Do We Begin?)" briefly segues into the opening theme.
In the G.I. Joe episode "Cold Slither" COBRA tries to brainwash audiences with subliminal messages in a rock band's music; the Joes foil the plot at a concert, and pacify the crowd by becoming The Average Joe Band. Guess what song they play?
The opening scene of Cosgrove Hall's The Wind in the Willows (the feature-length adaptation of the book, rather than the later series) has Mole humming the theme tune as he does his spring-cleaning.
The familiar themes to Merrie Melodies and Looney Tunes have been featured in several early cartoons with lyrics. "Merrily We Roll Along" was used in Billboard Frolics (1935) and Toy Town Hall (1936, both performed by an animated Eddie Cantor, who co-wrote the theme) before even becoming the Merrie Melodies theme. "The Merry-Go-Round Broke Down" (Looney Tunes) was a standard from 1934 and was performed by Daffy Duck with different lyrics in 1938's Daffy Duck and Egghead.
In the Thomas And Friends episode "Percy and the Calliope", after finally getting the calliope to work, it plays a remix of the show's original theme music (a popular callback for older fans as it had long been replaced since then).
In The Dreamstone, characters can sometimes be heard humming "Better Than A Dream". A polka version is also heard in the episode "Zarak".
The Adventures of Chip N Dale was a compilation special made in the late 1950s featuring a bunch of Chip 'n Dale shorts tied together with a catchy theme tune ("I'm Chip! I'm Dale! We're just a couple of crazy rascals out to have some fun!"). At the beginning of one of the shorts ("Donald Applecore"), Donald Duck starts singing this song to himself as he's heading out to harvest apples, only to catch himself and call himself an idiot.
The British Dennis the Menace's ringtone is an 8-bit version of Dennis & Gnasher's main theme
The closing theme of the original TMNT cartoon was frequently used as battle music on the show. Various other pieces of the show's music included their own snippets of the main theme.
In the 3-2-1 Penguins! episode "Practical Hoax", Zidgel hums the theme song when doing his hair.
In a Generator Rex two-parter where Ben 10 guest-stars, Rex, at the beginning, says he could use a theme song, and suggests one. However, instead of singing his own, he sings a new one to the tune of the original Series/Ben10 theme.
In the animated movie The Flight of Dragons, the tune that Giles plays upon the Flute of Olympus is the first couple lines of the opening song.
In The Cleveland Show episode, "'Til Deaf", Donna plays the title theme with a recorder when Cleveland leaves in a not-so hunting trip.
In Littlest Pet Shop (2012 TV Series), Blythe's ringtone is a MIDI/chip tune version of the cartoon's theme song.
In the Wild Kratts Season 2 episode To Touch a Hummingbird, Chris Kratt brought it up as his favorite theme song. Martin winked like he knew what Chris was talking about, played his bugle and, in turn, PLAYED THE THEME SONG. It was so catchy, Chris and Martin were actually swaying to the beat. Of course, being true to their personalities, their original intention was to wake up Aviva and Koki, which of course succeeded, and led to Aviva doing a Facepalm and Koki's Berserk Button to be pressed. It doesn't help that this was the beginning scene right after the theme song was played as per the usual with every episode.
There was also the ringtone in The Hermit Crab Shell Exchange, when Chris called the Tortuga. The ringtone played on the big computer spanning the whole damn wall, Koki's screen and Aviva's computer. The only difference between this and To Touch a Hummingbird was that it played the last four—I'm sorry, five—words of the theme song (Go Wild Wild Wild Kratts!) instead of the whole theme song.