The Circle (file system)

The Circle is a peer-to-peer distributed file system written mainly in Python. It is based on the Chord distributed hash table.
Development on the Circle was ceased in 2004.

Features

It supports file sharing, instant messaging with buddy lists, the Internet Relay Chat model of sending a message to a channel, and a personalized, trust-based news service.
More unusually, it supports:

finding and sending messages to people who aren't currently online;
a proxy for the Debian Advanced Packaging Tool system; and
a probabilistic mechanism for sharing common keys between nodes, something not present in all DHT implementations.

Details

Its DHT implementation is vulnerable to denial of service attacks.
From The Circle homepage:
The Circle currently runs on Linux, BSD, Windows, and Mac OS X.