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, one of my favorite languages with which to work.
(Update, 2006: I haven't touched quack in about 5 years. By now, the protocol implementation is probably horribly out of date.)
See quack's website for more info. Maybe I'll move some of it here later.