Windows/Mac: TorChat is an instant messenger client that makes encrypted, anonymous chat and sharing files 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 will come in handy if you need to transmit data or if you'd like to have a conversation without worrying about anyone sniffing your internet connection. When you sign into TorChat you'll get a random .onion address that serves as your ID. Share this with your friends and you'll be chatting privately right away. On top of the Windows version linked below you can get a Mac client, Pidgin plugin and a Portable Java version.