Many of us user Firewall, Internet security etc to safeguard our computer from external threat. PeerBlock is a similar application to safeguard unwanted internet connection specially P2P connections which many a times results in computer hacking when a security hole is left.
It gives you power to block governments, corporations, machines flagged for anti-p2p activities, even entire countries. Like I said about p2p it even blocks bad guys by checking the Bad IP by using lists provided by iblocklist.com
You can download PeerBlock from here and read more about it on their FAQ