OS X Mountain Lion Messages Beta
If you've read our OS X 10.8 Mountain Lion preview you know that the new Messages app is one of the spiciest new additions to the OS. And, good news for you, you can try it out right now -- if you've made the jump to 10.7 Lion, that is. It's available now for download, so get to clicking.

OS X Mountain Lion 10.8 in-depth preview