One of the better mash-up sites right now is The Hood Internet. The Hood Internet takes music from Indie bands and Hip Hop artists and just mashes them together. Despite the fact that the only similarity between these genres is that suburban teenagers are its biggest fans, the music is really good. Imagine The Go! Team and The Game on one track. How about Timbaland and Hot Chip? One of my personal favorites is the mash-up between Architecture In Helsinki and Snopp Dogg. Architecture's upbeat sound is a perfect compliment to Snoop Doggs lazy flow. The Peter Bjorn/john and Sisqo is so wrong that its amazing. It all just fits in its own contrapuntal way. You should really check out the site, the list of mash-ups is limited but its all good quality stuff.http://thehoodinternet.com/