PeerGuardian provides privacy-oriented firewall software. It blocks connections to and fro hosts specified in blocklists (thousands, if not millions) Its origin is in the targeting of aggressive IPs while using P2P.
PeerGuardian can be used as an open source project. Its source code is available for you to use, modify, and read. The project is also open for you and others to join and make contributions in any form (code and documentation, support, and bug reports).