TorChat, anonymous and secure messaging and file transfers
If you're on Windows, all you have to do is download, unzip, and run torchat.exe. It will automatically connect to the Tor network -- which will take a few minutes -- but then it operates just like a normal IM client. Once your name goes green on the contact list, you're good to go. You can add contacts (you will need their random string of 16 numbers and letters), and give them nicknames. You can even set an avatar and give yourself a short profile description. File sharing is simply a matter of dragging and dropping a file into a chat window.
Feature-wise, TorChat isn't remarkable -- instead, it's the simplicity and ease of configuration that makes TorChat so awesome. Because it's portable and your buddy list is stored in plain text, you can put yourself into the buddy list -- and then zip it up and put it on your website. If someone wants to communicate with you securely, they only have to download the bundle, run torchat.exe, and they'll be securely and anonymously talking to you within minutes. Read the TorChat site for more configuration info.
The Tor network is already used extensively for sensitive communication, such as whistleblowing, and with TorChat it makes the process a two-click affair. If you're in a line of work that requires you to keep your sources protected, TorChat is the best and easiest solution that we've seen.