Here are a few that I have bookmarked:

http://www.banjo.com/
http://www.nsknet.or.jp/~motoya/index.html
http://bluegrassworld.com/lyrics/
http://www.runet.edu/~bluegrass/songlnk.html

The other thing that's always good is to check out cdnow.com by artist or song title, and you can actually hear clips of lots of songs.

There are also a few right here in the DT categorized under @bluegrass, I think, and many others that are in different categories which I consider bluegrass.