Leon Kompowsky had a one episode guest spot on The Simpsons but the character is well known among fans and the history of the character's voice actors is an interesting story.
Leon Kompowsky appeared in the episode "Stark Raving Dad" (season 3 episode 1). Leon was Homer Simpsons's room mate in the mental institution he was sent to. When they meet Leon claims to be Michael Jackson.
Jackson performed the speaking voice of Leon Kompowsky under the pseudonym John Jay Smith. Jackson was a fan of the show and called Matt Groening one night and offered to do a guest spot.
The DVD commentary for this episode indicates that Jackson was not allowed to sing due to a contract obligation. Jackson also wrote the original song "Happy Birthday Lisa" for the episode.
One of Jackson's conditions for guest starring was that he would voice himself, but a sound-alike would receive credit.
While he recorded the voice work for the character, all of his singing was performed by Kipp Lennon. Jackson thought it would be a great joke on his brothers and fool them into thinking the impersonator was him. Jackson's lines were recorded at a session by James L. Brooks. Lennon and Jackson recorded their parts at the same time, and Jackson found Lennon's impersonations hilarious. Jackson actually did record versions of the songs, and while there have been rumors that those tracks were the ones used in the final episode, Simpsons music editor Chris Ledesma says the Lennon versions were used.
(To hear Michael Jackson's version of the "Happy Birthday Lisa" go to the youtube link provided below)
At the end of the episode "Michael Jackson" is revealed to be Leon Kompowsky and that he was only imitating the voice to make the world a better place.
The normal speaking voice of the character was voiced by VA Hank Azaria.
The producers of the show were legally prevented from confirming Jackson guest starred at the time, although many media sources assumed it was really him. After this episode the producers decided that if a celebrity wished to guest star on the show, they had to be willing to be credited under their real name and not a pseudonym.
Great secrecy surrounded the guest appearance for some time, and even Matt Groening, when appearing on the the Jay Leno show, did not openly reveal that it was Jackson (although he hinted strongly).
It wasn't until the episode "Itchy & Scratchy: The Movie" that a more open clue was given to viewers, when Lisa mentions that Dustin Hoffman and Michael Jackson made cameo appearances in the movie, stating, "They didn't use their real names, but you could tell it was them..." (Dustin Hoffman, of course, voiced Mr. Bergstrom in the episode Lisa's Substitute, and his guest appearance was similarly shrouded).
After Michael Jackson's death on June 25, 2009, "Stark Raving Dad" was dedicated in his memory.
So to sum up everything I just said.
Michael Jackson voiced Leon Kompowsky's "fake" voice.
Hank Azaria voiced Leon Kompowsky's real voice.
Kipp Lennon sang all songs in the episode for Leon Kompowsky ("Ben", "Billie Jean" and "Happy Birthday Lisa").