http://www.scatterchat.com/
ScatterChat is a HACKTIVIST WEAPON designed to allow non-technical human rights activists and political dissidents to communicate securely and anonymously while operating in hostile territory. It is also useful in corporate settings, or in other situations where privacy is desired.
edit: I don't know whats up with the illuminati symbolism on the programs page. However, this program does seem to be the most secure from the choices of chat programs available. It is also open source and open for inspection. Although, I have also become accustomed to thinking that anything transmitted on internet is in no way secure. This program just makes it extremely hard to intercept and decode communications. (You would probably have to be on someone's top 30.)