[eli]

For Mashups collections, check out Bootie: http://bootiemashup.com/ (and if you're in SF, definitely check them out at DNA Lounge). They do a downloadable annual Best Of album. http://bootiemashup.com/bestof/ (IMHO, the older albums are better.)

If you're looking for a full-album experience, check out the Kleptones. Here's an awesome HTML5 visualization of their album A Night at the Hip Hopera: http://mashupbreakdown.com/nhh

And, of course, Girl Talk