We (or at least Doc and I) love our mashups here at sandstorming. Contrary to popular ignorance, they were not made up by novaFM. There is a few good sites around but the latest one I stumbled upon through digg seems to be the best yet. It’s forum based, but registration is not required. Seeming though anyone can submit there is plenty of crap you have to wade through, but its worth it for the sheer amount of Gold tracks on there.
If you are looking just for some quality tracks, you can’t go past Dj Earworm. He’s the shit when it comes to mashups, and whilst he doesn’t have hundreds of them on his site, the quality is far superior to anywhere else.