DirectAccess was first introduced in Windows Server 2008 R2, and is greatly improved for Windows Server 2012 and Windows Server 2012 R2.
If you're interested in deploying DirectAccess, regardless of which of these operating systems you use, the new DirectAccess Survival Guide puts all of the Technical Library links to DA resources at your fingertips. Not only that, but the fact that this guide is on the Wiki means that you can add all of your own favorite links to DA information.