Slayers has between one to three image songs between each of the major characters (including one of the villains, Valgaav), and there is both the infamous "Maiden's Prayer" duet by Lina and Amelia's voice actresses from the second season of the anime, and a song where the four leads all sing together. The opening and ending themes (sung by either Masami Okui or Megumi Hayashibara, or both) are all Image Songs for heroine Lina Inverse, as they tend to describe her personality directly.
Sister Princess had an extensive soundtrack with solo and group songs for all 12 of the sisters.
Similarly for Mahou Sensei Negima!, featuring all thirty of the girls from Negi's class, alone and in combinations. The Opening Theme for the second anime features several re-recordings by different combinations of the girls, which were then used for the actual opening of different episodes. Many of the important characters (i.e. Ala Alba) are given their own songs as well.
One episode of Azumanga Daioh features some of the characters singing their image songs as karaoke. Interestingly, some characters turn out to be horrible singers in the "reality" of the show, whereas their voice actors are always at their best on the CDs (even when in character). In a later episode, when Yomi's listening to the radio, the DJ starts to play a request: "Kokoro wa Shoujo de Parachute" by Asakawa Yuu. This is Sakaki's image song and voice actor. They also use Tomo's image song, "Poi Poi Peace", as her ring tone.
Some of the earliest examples of image songs may well be found on the various soundtrack albums released for Bubblegum Crisis in the late 1980s and early 1990s.
The Di Gi Charat franchise is extremely guilty of this trope, with lots and lots of discs and singles released. Somewhat justified since the main characters want to be idols and the franchise itself born as an marketing strategy. There was even a crossover CD with Galaxy Angel featuring characters with the same VA.
Haruhi Suzumiya, thanks to its immense popularity put out 9 character albums, 4 soundtracks, 3 drama CDs, 8 combination soundtrack and drama CDs that shipped with one of the DVD versions, a live concert, an orchestral concert, followed by 7 more character albums for the 2009 re-airing from the Five-Man Band, plus Tsuruya and Taniguchi. All this for just 28 episodes! Emiri Kimidori, who only appears in a single scene for less than three minutes, gets a CD (although she later received an increased role in the later books that hadn't yet been adapted), and Churuya has 6 image songs. Naturally, the Gender Flip faction has redone even these image songs. And they're GOOD.
The Tenchi Muyo! series — or group of series variants — has released a whole lot of CDs, including a Christmas album. Characters have several songs that could be Image Songs, and some made their way into the first television series (which followed, and re-structured, the OVA), during karaoke on-board ship. A few of these were released in America by Pioneer, including one CD called "Meet The Tenchi Muyo" that mimicked the appearance of several Beatles albums.
Lucky Star has a large collection of image songs thanks to its overwhelming popularity. And it's not just the four main girls, either. Kagami's classmates and even Konata's deceased mother get songs. Misao, formerly one of Those Two Guys, became enough of an Ensemble Dark Horse to get an album all to herself. Shiraishi got more songs than anyone else thanks to him taking the lead for the second half of ending songs. One of the songs that got a full version was none other than Ore no Wasuremono.
The "Butler Network" segment advertises the show's image songs in a very direct, No Fourth Wall kind of way, including having Nagi wonder how well her song is selling.
Although not technically her character song, Hinagiku sings the ending of season two, it could fit for it though. Also for the second ending of season two, Hayate and Nagi sing, what could be considered, an image song.
Aside from the Season One mini-albums for the major characters (containing 2 songs, karaoke tracks, and interviews), there are a few albums for Season 2, The Movie, Can't Take My Eyes Off You, and Cuties with a few characters tracks for pretty much everyone - notably most characters got an Image Song for the ending of their personal episode in Cuties.
In a rare English language example, there is a soundtrack CD released titled Pokemon: 2B A Master that features Image Songs associated with characters from the show, but not performed directly by their English voice actors. Later, there was even a live theater show — Pokémon Live! — that featured the characters singing some of the songs from that soundtrack. The exception would be "Double Trouble", sung by Team Rocket's VAs. The Totally Pokémon CD released during Johto had a particularly memorable one sung by Brock's VA, Eric Stuart ("Two Perfect Girls").
On the Japanese side, there's Satoshi's (Ash's) themes "Spurt!" and "XY&Z" which are openings, Haruka's (May's) theme is "Watashi Makenai", Hikari (Dawn) has "Kimi no Soba de", Serena has "DoriDori" and Eureka (Bonnie) has "Puni-chan no Uta" (with all of their themes used as an ending theme of their saga). Instrumentals of these are usually heard during their contest appears. Team Rocket have "Rocket-Dan Yo Eien Ni" which they have occasionally sung in place of their motto. Takeshi (Brock) has "Takeshi no Paradise", originally an Ending Theme which he later started singing in episodes whenever there's a need for some kind or distraction. And Kojiro (James) has "Lucky Lucky", a flirtatious Hurricane of (Pokémon) Puns. Vocal versions of both these songs have made it into the dub.
Another rare English language example, (also courtesy of 4kids no less) is the Yu-Gi-Oh: Music To Duel By album which features "Face up Face Down" a theme song for Maximillion Pegasus sung by his voice actor Darren Dunstan.
Fushigi Yuugi had quite a few of these, most of the main characters had two image songs. A few of these were used in the series to punctuate serious moments i.e. character deaths. Notably, several of the image songs were actually background music from the show with lyrics.
Rurouni Kenshin had quite a few image songs, though they were not actually used in the show. These included songs for characters from mini-arcs.
My Bride is a Mermaid has a massive collection of image songs to its name, most prominently by San and Luna. Magic Music is such a major part of the show it was to be expected.
Ojamajo Doremi had four songs for each girl * Five for the original trio. by Dokkan, though Pop still only had two (and one wasn't even sung by her seiyuu). There are also two separate CDs featuring songs sung by their classmates and teachers.
The anime has image songs and singles for central characters in almost every season; only some of them actually appear in the show. There was also a CD of image songs sold in North America called "Lunarock!", which featured songs that appeared in a few episodes. Sailor Moon had more than one for herself on there, unlike common anime practice in Japan different voice actresses were used to sing for the girls, due to the album being recorded in L.A and the voice talent for the show being based in Canada.
Song Calendars and the "Hot Song Battle Contest" album display the voice actors singing wholly in character, sometimes purely for humor (such as Genma trying to sing while in Panda form), but for surprisingly dramatic effect at other times (girl-Ranma singing an unusually angsty song while crying in the rain).
The juxtaposition of Akane's two song really shows her Tsundere nature. Her infamous Baka Song shows the tsuntsun, while Akane's Lullaby is very deredere.
A little in-joke for the fans was the duet by Ryouga and the Jusenkyo Guide - the two characters share the same voice actor.
Magic Knight Rayearth often mixed image songs along with traditional soundtrack themes. Although they were usually performed by Hikaru, Umi, and Fuu, sometimes Emeraude and Eagle would get a chance to perform. Even Mokona got in on the fun with at least two songs.
A more extreme case is The Prince of Tennis, which keeps delivering image songs, duets, group releases and/or image CD's for almost each player in the show in a more-or-less regular basis. It's gotten to the point where the number of image songs is probably somewhere near four hundred.
Several of the Bleach characters have Image Songs released on CD, though they have never appeared in the anime itself.
Rosario + Vampire has a usual share of image songs in both its seasons, as well as covers of 80's J-Pop songs. In both seasons, all of these have been gratuitously inserted into the anime, with the poolside karaoke in season one and the singing in class in Capu 2 being the most obvious.
Several characters from Kinnikuman have one of these. Justified thanks to the cast being wrestlers, thus having ribg entrance themes.
G Gundam has five OST CDs, titled Gundam Fight Rounds 1-5 (though 1-2 are a two-disc set). Round 5 features image songs from most of the main cast, although while Domon Kasshu gets two songs of his own and a duet with Rain, Sai Saici and Argo Gulskii have to share "China Shuffle". And Argo's singing voice has to be heard to be believed.
Mobile Suit Gundam Wing has four OSTs, five if you include Endless Waltz; out of those, two are peppered with Image Songs, while a third is given over to them entirely (the only non-Image Song track being the show's second opening theme). Most of the primary cast gets songs, with the central characters (the Gundam Pilots and Relena) getting at least two while lesser-but-still important characters like Zechs, Treize, Lady Une, and Dorothy each get one. The 2-CD Audio PlayBlind Target features an additional song for each of the five pilots.
Mobile Suit Gundam SEED Destiny's second OST features "Shinkai no Kodoku", Stella Louisser's image song, which is also an insert song for her important scenes in the series, particularly her chance meetings with Shinn, her death scene and Shinn interring her body.
Both seasons of Hidamari Sketch have these, though much more extensively in the second season. Even the Principal gets to sing.
One Piece has quite a few image songs for the Straw Hats and for other noteworthy characters like Shanks and Mr. 2. Some of the Straw Hats' image songs have only one character singing, but there are often ones with multiple characters (for example, Hurricane Girls features Nami and Robin), or the entire crew. At one point during the Skypeia arc, Luffy and Usopp sing one of Sanji's songs. Badly.
Mai-HiME gratuitously inserted three image songs - one for each of the main players - into the show in episode 15. The show also got two full albums full of image songs for just about all of the main characters.
Ouran High School Host Club has some very fun image songs, as well as a song sung by all of the characters together. Noticeably missing is Haruhi's image song, and she only speaks in the group song. This makes sense as canonically, she's a horrible singer.
There are five Fullmetal Alchemist image song CDs, with three image songs each, one for Ed, Al, Roy, Winry and Hughes. And of course, there's the compilation CD with all of those on it, plus a few group songs. Sheska joins Winry for a duet (likewise Riza Hawkeye does one with Roy, and Hughes' daughter Elycia joins her daddy for a song that really Tastes Like Diabetes). The Homunculi manage to get in a really strange talk session set to trance music, when an Elric brothers duet probably would be a much better choice for Al's CD.
Meanwhile in America, Vic Mignogna (Ed's voice actor) wrote an unofficial tribute song in this style called "Nothing I Won't Give".
Saiyuki has numerous image albums from the three separate anime series and almost every major character has at least one song. And try not to count how many the four main characters have, you'll give up half way through. The only exception is Hakkai, since Akira Ishida doesn't like to sing—when Hakkai absolutely needed an Image Song as part of the "Four Seasons" promotional CD for the movie, he simply delivered a monologue to music. This has led original author Minekura to playfully refer to the group of Goku, Gojyo and Sanzo's seiyuu as "-8," or "Minus Eight," as the "ha" in "Hakkai" is written with the kanji for 8.
Pretty much the entire vocal soundtrack of Pani Poni Dash! is sung by the characters involved.
Standard practice in Lyrical Nanoha is to include three tracks worth of Image Songs in each Sound Stage as extra incentive to buy them. Nanoha and Fate naturally gets a good chunk of these, with Hayate a close third.
Music CDs exist for the eight main characters (or nine, since Italy's seiyuu does a song for North and South Italy). They also sing their own versions of the Ending Theme of the anime, which are included as extras on the DVDs. The endings for the first four seasons were all compiled into two CDs, and feature versions for other countries such as Poland and Lithuania.
The movie has "Wa! Wa!! World Ondo", in which the eight main characters all sing together, and the "Character CD Perfect Guide" gave Prussia his own image song "Mein Gott!" as well as a joint image song for Chibitalia and Rome called "Ren Ren Renaissance".
Then there are songs for Ukraine and Belarus, Prussia's other song "Song of the Awesome Me", "With Love From Iceland" with Iceland and Puffin, the digital-release songs for Hong Kong, Taiwan, Canada, and Rome (which were all put together along with two new songs; one featuring the Baltics, and another featuring France, Spain and Prussia in a commercially-released CD), as well as the songs from the Interval CDs featuring Prussia, Spain and the Nordics respectively.
In 2015, eight more CDs were released to tie-in to season six, featuring the main eight, Prussia, the Nordics, Estonia and Hong Kong.
The Rozen Maiden drama CDs all feature image songs to their respective dolls, sans Kirakishou, who doesn't have any drama CD.
The Hit Square Character Songs of Saki, a music CD containing the Image Songs of the two most popular characters from each of the four teams.
The anime Soul Eater has several albums of songs from the anime out along with at least three image albums out. The image albums each include one song (along with two other versions of the same song and a song from the anime not sung by the voice actors) sung by the Japanese voice actors. Soul Eater Evans along with Maka Albarn have a song sung together, Black Star and Tsubaki have a song sung together, and Death the Kid along with Liz and Patty Thompson have a song sung collectively. All of the tracks are quite awesome.
Tsubasa -RESERVoir CHRoNiCLE- has these for all of the major characters: Syaoran's being 'Kizuna,' Sakura's being 'Towa no Omoi,' Fai's being 'Smile,' Kurogane's being 'Zankou,' and Mokona's being 'Tabi no Tochuu de Kibou no Uta wo Utaou'. Though really, Yuui Makino (Sakura's Seiyuu) actually sings quite a couple of songs for the anime.
Katekyō Hitman Reborn! gives an image song to Gola Mosca, a machine who can't talk, much less sing. Instead he makes depressurizing noises from his exhaust ports which sound like creepy breathing. One of the Anime's endings was Sakura Addiction, a duet Image Song for Hibari Kyouya and Rokudo Mukuro.
Zettai Muteki Raijin-Oh is quite generous with its image songs; the three pilots and The Chick get two or three each, and the remaining 14 main characters each get a song. Even the villains get a shot in a "karaoke showdown" themed song.
Ronin Warriors has many soundtrack CDs, often containing image songs. Touma's singing voice is infamously bad and most of the others aren't much better - but a lot of the songs, especially groups, are fun anyway.
Dragon Ball has received many of these from over the years:
The Dragon Ball anime started the practice with the Red Ribbon Army arc, including "The Son Goku Song", "The Teachings of Master Roshi", and "Red Ribbon Army" for the villains themselves. "Wolf Hurricane" was also introduced and used for Yamcha's fight with Tenshinhan in the next arc. These supplemented several Insert Songs as well, such as "Aim to Be the Greatest on Earth", "Mystery Wonderland", "The Dragon Ball Legend", "With A Burning Heart! Defeat the Red Ribbon Army!" and "The Blue Travelers".
Strike Witches has image songs for the entire cast as well as many duets. With five CDs, covers by each girl for a song from their respective country from around that era, and a solo for each character (Yoshika got two solos) The ending theme for both seasons are sung by different cast members each episode as well. So far one of Sanya and Eila's duets, "Sweet Duet" has inserted into the end of season two's sixth episode.
Durarara!! came out with character singles for Mikado, Kida, Shizuo, Kadota, Anri, Shinra, Izaya, and Celty. All the songs are covers of old songs from anime or otherwise. For example, Celty's song is a cover of "Message in Rouge," the Japanese theme from Kiki's Delivery Service.
The Image Songs for Kannagi are the same ones from the Karaoke Box episode, although with improved vocals.
Dancougar has some sung by the voice actors themselves. Some of the notable ones are Ai Wa Zankoku No Fairytail and Ai Wa Shitta Yajuu No You Ni. The former was sung by the entire Juusenki Tai, the latter's just Shinobu.
Marginal Prince features a song by any of the characters in focus of the particular episode - episode Nine even features three of them. This was probably mainly done to promote the game the anime is based on, as it main feature is the songs as a "reward" for completing a character route. Luckily, mostoftheseiyuu are good singers. Unfortunately, not all songs from the game made it into the anime series (but therefore, some new ones were created).
Bamboo Blade: Each of the five main character got their image song. Sayako's song "Only Track" , sung by Sachiko Kojima was featured in the first soundtrack album. Kirino's song "For You Shine" and Tamaki's one "Sunflower", respectively by Megumi Toyoguchi and Ryo Hirohashi, in the second soundtrack. Miyako's song "All in all", by Houko Kuwashima, and Satori's song "No looser", by Rina Sato, where included in the drama cds.
Aria the Scarlet Ammo has a few, notably "First Contact" and "Ring a ding on, Riko-rin" for Aria and Riko
Kamina and Simon do a duet by the name of "Break Through the Dream." It is filled with about as much manliness as you can cram into a song.
Yoko has two "Trust" and "S.t.a.r.S.", the latter of which also has it's own full-length music video depicting Yoko's inner Idol Singer... and then Yoko with her inner Idol Singer. As can be expected from the series, it's actually quite awesome.
Spirited Away has an image album, unusually for a standalone film. Probably the highlight is No Face's oddly bouncy song about his loneliness.
Multiple major characters in Anpanman have an image song, most of them sung by their voice actors, with a couple sung by the musical duo Dreaming. A shortened version of Anpanman's song is the theme song for the whole series. For multiple characters, an instrumental version of the image song will be used as a leitmotif for their scenes. An interesting case is the Donburiman Trio: they each have their own song that they sing in-show, but their image songs are different (Tendonman's has slightly modified lyrics, Katsudonman's is tacked on to the very end of his image song, and Kamameshidon's is completely different). It's also common for very major characters to have more than one image song (Baikinman has one that glorifies him and another one that basically spells out his defeat).
Sword Art Online has them for Kirito and almost every female main character (Asuna, Leafa, Sinon, Lisbeth, Silica, Yui, and Yuuki), featured on the bonus discs included with the Blu-Ray sets of the series. Several of them have multiple cast members singing at once.
Tiger & Bunny has a few: "GO NEXT!!" and "My Song" for Karina/Blue Rose, "POWER OF JUSTICE" for Barnaby, and "Hamidashimono Sanka" for Kotetsu/Wild Tiger. "Seigi no Koe ga Kikoerukai" is a a duet with with Kotetsu and Barnaby while "Natsu no Koi wa, Otsukare Summer" features all three of these characters.
This is actually the theme of Senki Zesshou Symphogear. Every Symphogear has at least two image songs per season and at least one of them is used in the anime. This is justified because their powers can only be displayed when they sing. There are also some other songs and duets they do.
Mekakucity Actors, being the anime extension of Kagerou Project (a Vocaloid project) has a ton of these. Every character gets at least one (Takane and Mary get two, Momo and Hibiya get a joint-second), plus several other 'plot-based' songs. There are 23 songs in total, with one actually topping the 2013 Yearly Vocaloid Rankings.
Daily Life with Monster Girl has a soundtrack CD for each of the girls, each one consisting of two original songs, the featured girl's version of the anime's opening theme, and instrumental versions of the original songs.
Oddly, Black Butler has these. Characters who got songs are Sebastian, Finny, Grell, William, Lau, Undertaker, Agni, Soma, Viscount Druitt, and Ronald.