I've just stumbled upon one of the best mash-up albums I've ever heard; it's called "Cracked Pepper" by ccc / Ill Chemist which takes each track from Sgt. Pepper's Lonely Hearts Club Band in turn and mashes them with tracks by other artists making completely new songs while leaving the originals recognisable. Particular highlights for me are "With A Little Glam" which fuses "A Little Help From My Friends" with "Rock 'n Roll Parts 1 & 2" and the totally gorgeous "She's Slipping" which marries "She's Leaving Home" with Kinobe's "Slip Into Something". Wonderful.
Go grab it before some BPI/RIAA blowhard takes the tracks offline.