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.
http://music.metermaidsnyc.com/album/nightlife-in-illinoise

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?