For those not in the know, a mash-up is when two or more songs are combined to make a new song (i.e., the instrumental of one song, the vocals from another). Sometimes cheesy. Usually illegal. But there are some gems out there:
A Night at the Hip Hopera: The Kleptones have put this together and it is great. Lots of Queen mixed with pretty much everything under the sun. You can find a list of all the sources used here. Recently, Disney clamped down on this one, but you can still find mirrors of it here, here, and here.
Yoshimi Battles the Hip Hop Robots: Another mix by the Kleptones, but this one melds the instrumentals from the Flaming Lips' Yoshimi Battles the Pink Robots with a bunch of hip-hop a capellas (Beastie Boys, 50 Cent, The Pharcyde, Public Enemy, etc.). Quality stuff. I've only been able to find it via bit torrent (the link is here).
Bootmixed: This one's more of a party type one, it seems. Really, who can resist a combination of Nine Inch Nails and Ace of Base?
The Beastles: Ever wonder what it'd sound like if the Beatles jammed with the Beastie Boys? Here you go.
Jay-Z vs. _______: It seems that everybody and their mother has made a Black Album mash-up. stevevt over at DVD Talk has compiled a list of them over here.
That should get you through the weekend.