Yer into a niche territory here, because these sorts of bootlegs are often so rare or obscure that they half qualify as fan remixes! But if you mean tracks that have made it to 12" or a dj set rather than just on someone's harddrive, well okay...
i kind of like "Cowgirl on Ecstasy" - Peter Presta vs Underworld, which is in a Matt Josh dj set and on CDR
http://www.mixdepot.net/DJMattJosh/L...ril152006Part2
(Thanks to Lloyd for posting this a few months back...nice mix which makes Cowgirl seem harder than before, and a good set, too!).
Hate to admit it, but Fatboy Slim's mixing of "Born Slippy" with his "Right Here Right Now" (on "Live at Brighton Beach) and with Groove Armada's "At the River" (unreleased?) both work pretty well, esp. the former.
Lately, i also heard a mix with some samples that i am almost sure are from M.E. (the robotic 'gulp' sound), but it's not really a mash-up.