Many of the old file-sharing networks are still around and actively in use. MuWire has a lot of interesting books and recordings. EMule is a good place to find music, including obscure remixes. Gnutella is mostly porn, including child porn that's so open I feel like it might be part of a law enforcement operation.
Retroshare seems like a p2p Facebook rather than a file-sharing network. I've always wanted to get into it, but I don't know anyone else using it.