I recently discovered Mashuptown, which is sort of an aggregator of various mashups around the web, mostly with a bias toward the strictly dance-oriented stuff. Nothing wrong with that, but nothing I've heard here tops the scope, humor, sophistication, and general magnificence of "A Night at the Hip-Hopera."
Still, there is some good shit, so to speak. My favorite track of what I've heard so far is "Smells Like Booty" (posted last May), which wins points as much for its title as for its musical content: a combination of two particularly monstrous (in more ways than one) hits by Destiny's Child and Nirvana. (By the way, April Winchell recently had a hilarious post that catalogued many of the too many covers of "Smells Like Teen Spirit"-- one of which finds the tune decked out in a gregorian chant style (April's commentary: "smells like nap time").)
Also worth checking out: "Jim, Can You Tell Me How To Get To Sesame Street?" (The Doors plus the Macarena plus the Muppets.)