Off the Record -- deniability amd perfect forward secrecy
Nikita Borisov and Ian Goldberg have released Off-the-Record Messaging. This is a really cool IM plugin for private communication. It goes beyond the usual encryption and authentication of GPG. It also deniability and perfect forward secrecy. Deniability means that no one can prove that a message is from you, while still maintaining authenticity for for the intended recipient. Perfect forward secrecy means that your past messages and conversations remain protected even if your keys are compromised.
I've read their paper and I'm very impressed. Their system is very clever, while being based entirely on tried-and-true technology (DH key exchange, HMAC, etc). If you are interested in secure communication, I do recommend you take a look at this. They have a GAIM plugin and an LGPL library.