Outside of LiveJournal and Chuzzle, my wife uses Windows Messenger more than anything else while she's sitting at our old, outdated, slow PC. Messenger is pretty cool, and Office Communicator is pretty average, but the new Windows Live Messenger looks awesome.
Check out a video over on The Show On 10 and then rush out and download it.
In a related note, it looks like Microsoft is teaming up with Disney to release a special "Dead Man's Chest" movie tie-in game that has something to do with Messenger. I haven't explored this yet, but it looks pretty cool on the surface.
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