PeerGuardian 2.0 Beta 6b
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.
Development of this title has been discontinued by the original authors, but has been continued by a new team under the name of PeerBlock.