Simple tool to configure Windows Filtering Platform (WFP) which can configure network activity on your computer.
simplewall vs PeerBlock opinions
Completely different approach but with similar results
iplist is a Linux application for blocking connections to and from a specified range of hosts using the netfilter netlink-queue library. iplist is an open source IP filtering program similar to PeerGuardian .
The project is no longer developed. Last version, 0.29, released in October 2010, can be still downloaded from the official website.