Collaboration means working with someone else, which is far too broad a definition to make a list. I guess what we're looking for here is solo acts with other solo acts? Though bands are in fact people working in collaboration, they have to be excluded, therefore I choose to exclude 'super-groups' as well.
Now, I'll proceed with breaking these rules as necessary.
Favorite Song Collaborations
10) Walk This Way by Aerosmith and Run DMC - Kinda had to be here, right?
9) Hunger Strike by members of Pearl Jam and Soundgarden - Technically these guys were called Temple of the Dog but it can't be a super group because no one knew them yet. Weak defense! Rule broken!
8) The Next Time I Fall by Peter Cetera and Amy Grant - Didn't I tell you to shut up?
7) Young Americans by David Bowie and Various Artists (including Luther Vandross) - I guess I mean the song, but I kind of mean the whole album. For his ninth studio album, Bowie said, "Fuck this, I'm going to Philly," and he did, and worked with many rad dudes to make his soul record.
6) (I've Had) The Time of My Life by Bill Medley and Jennifer Warnes - When I was young, I thought I liked Dirty Dancing less than everyone else. Turns out, that fucker keeps popping up on my music lists.
5) Don't Go Breaking My Heart by Elton John and Kiki Dee - You know, Kiki Dee! The famous one of the pair.
4) Don't Let the Sun Go Down On Me by George Michael and Elton John - Ah, shit. George Michael is dead.
3) Say Say Say by Michael Jackson and Paul McCartney - Their fallout was legendary, but this came from their partnership.
2) While My Guitar Gently Weeps by The Beatles and Eric Clapton - I decided to leave 'guest spots' out of the list, but this is the greatest one ever, if it even qualifies. (Also, to be specific, this is a George Harrison-penned song.)
1) Under Pressure by Queen and David Bowie - This has to be at the top of the list because it's one of the best Queen songs and one of the best Bowie songs.
Short Answer: Apparently, hip hop artists collaborate a lot. Who knew?