Microsoft Lync 2010 replaces Microsoft Office Communications Server for workplace collaboration
The major new stuff in Lync 2010 includes enterprise voice calling that's compatible with various USB and IP devices, and could theoretically do the job of your office's legacy phone system. Contact lists are now unified across your Microsoft applications, so you can use your contacts in both Lync and your office apps. Lync also includes new conferencing features, like desktop and app sharing (including PowerPoint sharing and whiteboarding).
The last big Microsoft OCS release was back in 2007, although it got a service pack last year. Lync 2010 represents a huge leap forward in functionality that looks like it actually belongs in a 2010 office environment. There's an insane amount of info on new features, including several demo videos, at the Microsoft Lync 2010 launch page.