giFT will connect you to the OpenFT, Gnutella, and FastTrack networks. There might be more plugins, but I am not aware of them. It can be found here:
http://gift.sourceforge.net. You will also need a client and at least one network plugin (they can all be found on that site). My personal favorite, however, is museek, which can be found at
http://museek.thegraveyard.org. It will put you on the Soulseek network. Museek is still very young, only at it's second release, and lacks some features as of now. Nicotine is an alternative to museek:
http://nicotine.thegraveyard.org, it is older and rich with features, but a LOT slower than museek. If you want a good BitTorrent program, I reccomend Azureus (
http://azureus.sourceforge.net). A safepeer plugin is also available at that site which will block the RIAA, etc. Java apps normally piss me off, but not Azureus. Be warned, though, that using the safepeer plugin will cause Azureus to take MUCH longer starting up. Well these are the ones that I've tried, I hope this information is useful to you.