Microsoft Messenger for Mac 8 is out of Beta, available to download as a standalone app
Messenger for Mac has all the features you'd expect -- it lets you instant message, audio and video call your friends who also have a Windows Live ID. Naturally, a Windows Live ID is required to use the application, but you already have one of those if your email address ends in live.com, hotmail.com or msn.com.
This release is targeted at people who use Office applications on the Mac. Thus Microsoft chose to emphasize how Messenger can help with productivity, by allowing real-time collaboration between Mac users and their Messenger contacts. Because of this, certain features that are part of Windows Live Messenger 2011, such as games or social views, will not be available for the Mac version.
Microsoft Messenger for Mac 8 is free and can be downloaded from here.