Microsoft has released the Kinect for Windows SDK beta, as expected, allowing PC developers to use the motion-tracking accessory. A free 100MB download, the SDK offers support for the depth sensor, color camera and quad-microphone array, along with all the clever skeletal-tracking systems that Xbox 360 game developers have had access to.
There's also integration with Windows' speech recognition API. That potentially means developers will be able to use the microphone array - which can pinpoint which user is talking thanks to beam formation - to transcribe speech to text, open and control applications, and more.
Microsoft has thrown in plenty of technical documentation, samples, all the drivers you'll need and support for C++, C#, or Visual Basic. This current iteration of the SDK is only for non-commercial purposes; Microsoft says it will be releasing a commercial version later on. You'll obviously need a Kinect sensor, too, which currently costs around $140.