Back in December of 2011, Microsoft released the first version of Lync for Android, which brought real-time Exchange collaboration to mobile. Considering that version came before ICS and the Android Design Guidelines, it looks a little (read: very) outdated on modern smartphones. Thankfully, Microsoft just released Lync 2013, which brings a nice looking Holo-meets-Windows-Phone sort of UI along for the ride.
The app essentially retains all the functionality of the previous version, allowing you to IM, collaborate, and video chat with other colleagues using Lync. It's noted in the Play Store that some features may require Lync Server, though it doesn't make specific mention of what those may be.
The app is a free download, though a "validly licensed copy of Microsoft Lync Server or Office 365/Lync Online" are required in order for the app to function. But you probably already knew that.