Windows Live users will continue to use Messenger for Mac and Office Communications Server (aka Lync) users will start using the new Communicator for Mac

Messenger for Mac will continue to be a free download for Mac users that want to access their Windows Live network of contacts. The focus of our team will continue to be features that extend the productivity of Windows Live users who use Office applications. Features that fall outside of that mission, such as games or winks, are not likely to be supported.

Communicator for Mac will be available to organizations that use the Office Communications Server. These enterprises, companies and education organizations will provide Communicator for Mac to their users as part of Office 2011. (Communicator for Mac will be available to volume licensing customers only.)

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http://blog.officeformac.com/announcing-communicator-for-mac/