From their website:
http://retroshare.sourceforge.net/RetroShare is a Open Source cross-platform, Friend-2-Friend and secure decentralised communication platform.
It lets you to securely chat and share files with your friends and family, using a web-of-trust to authenticate peers and OpenSSL to encrypt all communication.
RetroShare provides filesharing, chat, messages, forums and channels
I you want to use it with me:
download/install
create identity
add my key, above
post your key, I will add it to my installation
Then we get to see the power of retroshare.