Rather than necro-ing a zombie thread:
I’ll just start a new one.
Phil Collins vs Kill Bill
Blondie vs The Doors
ELO vs The Supremes…plus a little Jacko
And a link to another message board:
http://community.discovery.com/eve/forums/a/tpc/f/9801967776/m/77819493301
Kill Phil
“Sorry, Phil.”
Queen vs Genesis
Wow. I think the Battle Without Honor or Dignity/In the the Air Tonight mashup might be the most awesome thing I have ever heard.
The tv show Glee had a whole episode centered on mashups, and have used them in another couple of episodes. They are performed by the cast, though, so not mashups of the original recordings.
Here are some of them via youtube:
Don’t Stand So Close To Me/Young Girl
It’s My Life/Confessions Part II
BTW, on those last two there is a reason within the context of the show as to why the kids are so… animated.
And if you’re not watching Glee, you really should–I didn’t think it would be my kind of show, but I love it.
Can The Rolling Stones’ “Memoy Motel”
be mashed up with U2’s “One”
I’m no musician, so I don’t know from chord progression, but I figure you can speed up/slow down one song to match the beat of the other, and run one song through a sound filter to match the key…much as was done in this mashup:
Best of Bootie 2009 just came, the other ones were the best of the last mashup thread. Bootie - Best of Bootie 2009
DJ Earmworm just did a mashup of the Billboard Top 25 tracks for 2009.
See how many you can spot. 
I’ve got six songs simultaneously stuck in my head now.