Peer Block is a new version (aka a "fork") of the popular Peer Guardian 2 software. It blocks "known bad" computers from accessing yours, for example governments and corporations. Peer Block maintains the functionality of the original Peer Guardian 2 program, but includes fixes for various Windows Vista / Windows 7 issues that remain unaddressed in PG2.
Note on License: Please note that the Peer Block source code is NOT necessarily released under the GPL, regardless of what the little sidebar says. The original source code for Peer Guardian 2 was licensed under a "zlib/libpng License", and I'm not sure if I'm allowed to relicense it under the terms of that license. Google Code does not however allow "zlib/libpng" as an option, but as the PG2