I only recently came across the metermaids mash up with sufjan stevens despite having both artists in my library. Its pretty sweet. and free to boot.
I know of
Wugazi album (Wu-tang/Fugazi)
and Dj N'WeesSlack album (Jay-Z /Pavement)
Gray album (Jay-Z / Beatles)
Black and Blue album (Jay-Z/ Weezer)
Jay-z mixed with modest mouse /radiohead etc
what other good Mash Up's are out there?