Windows/Mac/Linux (All platforms): Alliance, a free, open-source, cross-platform peer-to-peer application, takes nearly all of the security and privacy concerns out of peer-to-peer file sharing by putting you in charge of your own network. The dead-simple interface lets you add Alliance-using friends to your network and files on your system to share, and you can search, chat, and download like any other peer-to-peer app. The traffic between clients is encrypted at a low level, but you can apply an experimental SSL layer if you’d like a bit more protection from snooping. For trading files with co-workers or friends, it’s a nice no-overhead solution. Alliance is a free download for Windows, Mac OS X, and Linux systems.