Resources for Mashups and stuff..

Free / Open sound resources:

Beatport <http://www.beatport.com/>,
Bleep <https://bleep.com/>,
FreeSound <http://freesound.org/>,
Acapellas4U <http://www.acapellas4u.co.uk/>,
ccMixter <http://ccmixter.org/>
DJServicePack <http://djservicepack.com/>
PromoDJ <http://Promodj.com >

Other places to check out:
http://www.reddit.com/r/SongStems/
Unfortunately, I'm not sure about the legal status of most of these stems...

Echo Nest sandbox with stems of Jethro Tull <http://developer.echonest.com/sandbox/emi/jethro_tull.html>

Bands like Nine Inch Nails, MuteMath, Zorch, Trifonic and others releasing freely their albums as stems, to allow people do their own mix, mashup, etc.

Ableton has sets that are open for reuse <https://www.ableton.com/en/blog/tags/able10/>

You may also want to have a look at Indaba Music, in particular their remix opportunities (small fee to enter for one year). They organize remix contests on a regular basis, and for the majority of these, you can obtain stems for the full songs.