Nirvana vs. Europe
Concur - very well done.
I doubt it. He liked Michael Jackson.
Lots of these are great.
He wasn’t fond of the commercialization of his own work though.
Yep, one of my favorites, if not my favorite mash-up (and I’m generally not a fan of the genre.)
Dexy’s Midnight Runners and Public Enemy
Bring the Noise with Come on Eileen
Doesn’t this just prove that our parents were right when they said all of our music sounds the same?
An oldie, but a goodie! 
My favorites:
Iron Maiden vs. Frankie Goes To Hollywood
Rock Sugar: Don’t Stop the Sandman
And while not technically a mashup, this cover of Duran Duran’s “Rio” has a cute little medley at the end.
Lady Judas (Lady Gaga vs Judas Priest Mashup by Wax Audio)
Justin Bieber vs. Slipknot - Psychosocial Baby
PSY vs Ghostbusters - Gangnam Busters - Mashup by FAROFF
I like this one:
The Beatles vs Fatboy Slim - Walrus of Choice with bonus of C. Walken’s mad dance moves.
If You Kiss Jesus, a mashup of a number of songs that sounds awesome.
Thunderstruck and Ghostbusters
FXMastermind: Oh, god. There goes the rest of my day. 
Chimera: Where can I get one of those Cthulhu-Aid T-shirts?
The mashup of Beatles songs"Within You Without You" + “Tomorrow Never Knows” from the Love album is among my favorites. There is a little bit of backmasked “Rain” and “Lucy in the Sky w/ Diamonds” in the outro as well. The mix was performed by George Martin and his son Giles Martin.
Mash-ups work better when the key and tempo match. When the speed of one or both songs is manipulated as evidenced by some of those already posted.
I think my favorite is “Disturbed Youth” which is Beck vs Rihanna.
I also quite like “C’mon Fuck Me” Princess Superstar vs. Simian vs. Von Bondies vs. Fatboy Slim
They don’t quite play fair with this mashup of Queen and the Beatles, and yet it’s still perfect.
Holy crap, this Lobsterdust guy is good. Nelly vs. the Bee Gees: Stayin’ Hott!
