P2P Opensource

eMule
eMule is a filesharing client which is based on the eDonkey2000 network but offers more features than the standard client
http://sourceforge.net/projects/emule/

BitTorrent
http://sourceforge.net/projects/bittorrent
BitTorrent is a tool for distributed download. Go to http://www.bittorrent.com/ for the latest version, or http://www.bittorrent.com/dl/?C=M;O=D for source and older versions.

DC++
This is a project aimed at producing a file sharing client using the ADC protocol. It also supports connecting to the Direct Connect network.
http://sourceforge.net/projects/dcplusplus/

Ares Galaxy
Chat/Filesharing p2p client. Network is organized into leaf/supernode topology featuring broacast-type searches. Ares delivers broader search horizon by means of DHT technology.
http://sourceforge.net/projects/aresgalaxy/