Well, music is what people share, it's not built into the program.

There are many decent gnutella programs around. Phex, the 'early' Bearshare 5 version, ShareAza, Morpheus, Gnucleus.

If you mean p2p in general, there's several p2p sharing networks. Some of the above programs can access more than one network. Then there's torrents. And also direct connect which used to be great.