Here are a few mashups/bootlegs/bastardizations I'm enjoying immensely just now..
Challaback Girl - this frail white girl from California has been Hindu, Japanese, and gangsta.. why not Jewish? djBC keeps Gwen Kosher.
Q Unit - Queen meets 50 Cent, makes 50 Cent actually listenable. A full-length album of goodness.
Jesus Walked Back and He's Black - Kanye West and AC/DC.
The Ciccones - The Immaculate Concoction - Full album of Madonna vs everyone.
Burn Your Radio - Public Enemy meets Queen, with other good stuff layered in.
Supercalibreakz - Supercalifragilistic(etc.) layed over "Rolling" by Shy FX, fun in that way only bastardized D_sney can be.
Come Undone vs Soldier - Duran Duran can even make Destiny's Child listenable.
For much more, check out GYBO!
Speaking of GYBO, there's an awesome and huge video file going around - the Club GYBO Plan B mix by Thriftshop XL - lots of video-mashes, remixed video and audio re-remixed together, and just plain video insanity in a non-stop 45-minute-long mix. Seriously awesome, check it out of you have the bandwidth and disk space to handle a 133 meg WMV file.