Office 2010 Professional Plus includes Communicator. The official version number is actually Office Communicator 2007 R2 and it is an enterprise messaging services. Thinking MSN Messeger on steriods and in a enterprise environment. If you're running Exchange Server and Office Communications Server this is a must have.

Here are some new features that are loaded into integrated version / product. You can also learn more on Microsoft Office site.

Collaborate with external organizations (not on your enterprise server) to join a conversation that you are hosting, share your desktop with a vendor to discuss a project, or start a conference call with a stakeholder and route the call to your handheld device—all with only the Web browser.

Desktop sharing. thinking Windows Live Meeting.

"Dial-out” conference audio. Audio conferencing (using standard telephones, including cell phones) can be added to any existing instant messaging or desktop sharing session. Think gotomeeting.

Support for distribution groups. You can add distribution groups to your contact list and exchange instant messages with all (or some) of the members of those groups.

This and a whole lot ore. I highly recommend this for any business.