LimeWire self proclaimed the Fastest P2P File Sharing Program on the Planet is an open source app Features:
*Ability to search by artist, title, genre, or other metainformation
*Elegant multiple search tabbed interface
*"Swarm" downloads from multiple hosts help you get files faster
*iTunes integration for Mac and Windows users
*Unique "ultrapeer" technology reduces bandwidth requirements for most users
*Integrated chat
*Directly connect to a computer
*Browse host feature--even works through firewalls

Maybe the folks at LimeWire should have considered the terms of the GPL before charging for their app, but Webby's World has the details on acquiring LimeWire Pro for free. Or just go to the main site and down it... for free.