Aqil Davidson: Still in 1991, Teddy Riley produced the Dangerous album track "She Drives Me Wild." So unsurprisingly, he put his Wrecks-N-Effect boy Aqil on for a quick verse in the middle of the song. He raps to the girl whose look is driving Michael wild ("far from Medusa"), and says, "you've got me lookin' like Buckwheat." Whatever that means.
Shaq: When you look at Mike's list of chosen hip-hop collaborators, I think one word will spring to mind before all others: "integrity." And when you're all about the art, putting aside the publicity and the image to only work with the most talented, best of the best, there's one man you go to before all others. If you're organizing a basketball game. But for some reason, Mike got confused and asked Shaquille O'Neal ("the man of steel organism") to rap for him instead. So he kicks a little verse at the end of "2 Bad" from HIStory, including his signature fake laugh.
John Forte: In 1997, Micheal's people put out a remix EP called Blood On The Dance Floor - HIStory In The Mix. One of the featured tracks was the Refugee Camp Remix of "2 Bad" (which was on the original version of HIStory), which now included vocals by John Forte.
Update 12/4/15: Hey, I noticed this post is riding up the "Most Popular Posts," column, and I thought I should update it with what came out since this was written back in 2009. They've put out more posthumous releases, with guest appearances by guys like Akon and Justin Beiber. But just one more with a rapper...
50 Cent: The album Michael, released in 2010, featured all unheard Jackson songs including one called "Monster," with raps by 50 Cent. It's clearly an unfinished song and filled with vocals from other Jackson songs to flesh it out, and there are rumors that other parts were sung by an impersonator. But nobody had to fake the verse from 50, who comes in with an odd mishmash of bragging about how hard he is, scary monster movie imagery to stay on theme, and just generally heralding the return of MJ. There was even a single for "Monster" in 2011 with a terrible house remix.