Mashups, because mashups are cool.
Outkast vs. Queen - Hey We Will Rock You (Hey Ya vs. We Will Rock You) - amazingly dancable and it's just plain awesome on so many levels. One of my all time favorites.
The Clash vs. The Killers - Somebody Rock Me (Rock the Casbah vs. Somebody Told Me) - Also quite funky and fun by two of my favorite rock groups.
Kelly Clarkson vs. Led Zepplin - Behind These Immigrant Eyes (Behind These Hazel Eyes vs. The Immigrant Song) - I'm irrationally drawn to the really bizarre mashups, and this is one of them. It's really good, all the more so for being able to mashup up Kelly Clarkson and Led Zepplin.
Christina Aguilera vs. The Strokes - A Stroke of Genius (Genie in a Bottle vs. Hard To Explain) - A true classic. Makes Christina Aguilera listenable.
The Soggy Bottom Boys vs. Gwen Stefani - Constant Sorrow Hollaback Girl (Man of Constant Sorrow vs. Hollaback Girl) - Weird, interesting and cool. It's fun just listening to see how they mesh up.
And just for xaeris and bursar42,
All Your Base Rhapsody - (All Your Base vs. Bohemian Rhapsody) - Just watch it. No really. Just watch it.