(Update, 2006: I haven't touched quack in about 5 years. By now, the protocol implementation is probably horribly out of date.)

Quack is a server-only gnutella implementation. It is intended for use by people who are interested in sharing full time -- those who have a permanent connection to the internet. Quack still needs some work -- it doesn't actively maintain a connection to the gnutellanet. But it responds to searches, performs some (not very intelligent) routing, and will upload files upon request. Quack is written in python.

See quack's website for more info.