PeerGuardian 2 is Phoenix Labs' premier IP blocker for Windows. PeerGuardian 2 integrates support for multiple lists, list editing, automatic updates, and blocking all of IPv4 (TCP, UDP, ICMP, etc), making it the safest and easiest way to protect your privacy on P2P.
PeerGuardian 2 is Open Source, meaning the program code is available online for anyone who wants it. Not only does this allow you to make modifications or even branch off into your own project, but it encourages peer review of the code making sure any bugs are eliminated swiftly. Because PeerGuardian 2 is Open Source, you can rest assured there are no backdoors or spyware included.