PeerGuardian Lite
PeerGuardian Lite is a derivative of PeerGuardian 2 made to consume as little CPU and RAM as possible. It has no UI or options and consists of a single tray icon. It is no longer developed, with the latest version released on April 22, 2005. It is also open source, allowing for future derivatives by any party. As its developers reported online, Peerguardian Lite is now continued under the project and program Peerblock, and Peerguardian Lite users were recommended from around 2008 on to switch to Peerblock. Peerblock has some more options, but uses almost as little CPU as Peerguardian Lite did.
Read more about this topic: Peer Guardian