PeerGuardian is a privacy oriented firewall application. It blocks connections to and from hosts specified in huge blocklists (thousands or millions of IP ranges). Its origin seeds in targeting aggressive IPs while you use P2P.
PeerGuardian is an open project. Not only is its source code open for you to read, use, and modify - but the project is open for you to join and contribute in any form (code, documentation, bug reports, web and support).