Most popular P2P file sharing software in 3 years ago.

1st one is Winny.
English startup guide:
http://www.uguu.org/winny/

Wikipedia entry:
http://en.wikipedia.org/wiki/Winny

New one is share.
English localization and startup guide:
http://www.uguu.org/share/

Wikipedia entry:
http://en.wikipedia.org/wiki/Share_%28p2p%29

Important notice!
Police already watch that network.
Be careful.