PeerGuardian is designed to block the IP addresses of those individuals that might want to do you harm and prevent you from accidentally or intentionally connecting to them or them to you. Advertisements, spyware, government, and education ranges as well as IP blocklists that typically contain those RIAA/MPAA trolls or MediaDefender folks.
Even if you’re doing nothing bad, it doesn’t hurt to block out those that are intrusively waiting and watching for you to make a mistake. PeerGuardian, developed by PhoenixLabs, also blocks those things you typically want to keep your distance from anyways, like spyware or malware.
Speaking from experience, PeerGuardian2 runs in the background very quietly using hardly any resources. The only time you’ll think twice about PG is at start up when it updates its list, but that’s it.
An interesting article from ArsTechnica, that cites some research about the effectiveness of PG2.
Download PeerGuardian, I would if I were you.