Today Messenger for Mac 8 came out of beta! It will be available (soon) as part of the Office for Mac 2011 suite, and is also available as a free standalone download.
Messenger for Mac 8 is free and provides real-time collaboration between Mac users and their Messenger friends. You can use instant messaging, audio, and video calls to communicate with your Messenger contacts by signing in with your Windows Live ID.
Games, winks or the new social views available in the Windows version of Messenger 2011, won’t be available for the Mac.
More information available here.