Packt Publishing are once again offering a free computer programming title (DRM-free ePub/Mobi/PDF bundle & online reading in most cases) per day until May 17th.
Here's the promo page which will showcase them all (they're operating on GMT and rotate daily ~ 4 PM Pacific Time), and you'll need to already have an account with them to claim each title to add to your library:
Free Learning — Help Yourself, and today's freebie is for
Kinect in Motion – Audio and Visual Tracking by Example