RetroShare is a file sharing and communication platform that allows you to create your own decentralized file sharing and communication network. Unlike other file sharing applications, RetroShare only allows people into your network who you exchange GnuPG certificates with (a certificate is created for you when you set up the server). RetroShare enables you to share files with your authenticated friends in complete privacy, all communication is encrypted and nobody else (not even your ISP) can see what you are sharing.
In addition to sharing files, you can also exchange messages (with optional attachments) with your friends from an email-like communication client or the built-in messenger. Other features include network search, UPnP / NAT-PMP port forwarding support, built-in forums, plug-ins and more. RetroShare is a full featured and powerful sharing platform however it is not a "file sharing for dummies" application. Although the software is designed to be very user friendly, it requires you to be comfortable with some basic networking concepts. RetroShare is portable, no installation needed.