Radiohead is one of the most well-known bands in the world, and they just love having their music remixed. Check out their site (http://www.radioheadremix.com/) regularly for tweaky strange sounds and lilting English voices:
Indaba music (http://www.indabamusic.com/home) is relatively new on the scene, but has a regularly awesome set of artists who are willing to let other artists take a crack at their work. They also provide a networking solution for artists who are looking for producers, remixers, and so on. Their roster of talent is impressive and they also have sample packs available for download:
Remix comps (http://www.remixcomps.com/) is a site that is always on the lookout for who is putting a remix contest out in the world. It's certainly more of an aggregate site, but one that has found remix contests for everyone from Alicia Keys to Kaskade:
CCmixter (http://ccmixter.org/) is another great site that actually has public domain sample packs for downloading and using. They are using a different type of copyright, creative commons that allows for other artists to use the sampled works free of litigiousness. They also have a community of remixers and artists, so it can be a great place to find other like minds in the electronic music field:
