I've gotta put in a vote for the Gnutella network; specifically, BearShare . While it's not as intuitive as KaZaA or other programs, it has some features that they don't have: primarily, the ability to connect to specific other users. It uses bandwidth pretty well, and runs without sapping too many system resources. It also is run without any "centrality," which means no usernames, email addresses, or the like.
If you disagree, however, I've found that
Zeropaid.com is a pretty good place to start when looking for a file sharing utility.