Using darknothing.com

1. Arrange secret pass phrase in person with friend to be used as encryption key.
2. One of you "Create Room".
3. Pass room # to friend via other method - FB chat, Twitter IM, text, whatever.
4. Friend "Join Room" using number provided.
5. Click the settings cog to set screen name and encryption key.
6. Chat.
7. One of you close the chat window. Your messages in the other client should be removed (replaced with --HAS LEFT THE CHAT--)
8. Other person closes chat window.

Quick demonstration