pDonkeyS is Perl implementation of an eDonkey server. The main goal of the server is to provide framework for further eDonkey protocol exploration, but it could be useful in testing of client software, since the server can be configured to output very detailed debugging info about eDonkey traffic.
Credence is a tool for combating spam and pollution in filesharing networks. It lets you vote on files in the network, analyzes the votes of your peers so that you can avoid polluted files, and automatically identifies the voters in the network that are most credible and useful. It is built as an extension to LimeWire, running on the Gnutella filesharing network.