Saw this on Kottke, it's amazing. Follow the links for more a capella: [youtube=http://www.youtube.com/watch?v=VrAdX4O1m4M&w=700]
Many more isolated vocal tracks are available on this subreddit. Here are the instructions for making your own isolated vocal tracks with Audacity, the same open source audio processing app that Tim used to make his slow jams. (thx, tim)