Windows/Mac: TorChat is an instant messenger client that makes encrypted, anonymous chat and file sharing with your friends incredibly easy. Built on Tor's location hiding services nobody will be able to see what you're doing or who you're contacting.
While most of us won't need TorChat too often, it's bound to come in handy at some point when you need to transmit data or if you'd like to have a conversation with a friend without worrying about anyone sniffing your internet connection. When you sign into TorChat you'll get an random .onion address that serves as your ID. Exchange this with your friends and you'll be chatting privately right away. On top of the Windows version linked below you can get an OS X client, Pidgin plugin, and a Portable Java version.
TorChat | via One Thing Well