Mac: We're fans of the Cryptocat web app for creating encrypted chatrooms in any web browser. Now, the private chat client is available on your Mac.
Every chat you have on Cryptocat is encrypted, and nothing is saved on a server. When you first load it up, you'll create a room, and you get a numeric key that you can share with anyone else you want to join the chat. After that, you can chat with the confidence that nobody is snooping in on you. It's simple encryption that anyone can use.
The desktop version also gets notifications and sounds, and since it doesn't need to run in a browser you don't need to worry about encryption on a browser level.
Cryptocat (free) [Mac App Store]