To celebrate the company turning 30 years old, all users of Ubisoft's Uplay PC client are recieveing monthly gifts these days. The first game they gave away was Prince of Persia: The Sands of Time, before they moved along to the original Splinter Cell.
That game is still on offer completely free, but come August 17th Ubisoft are switching over to Rayman Origins. This will contiune throughout the rest of the year, so if you want to catch all the free games be sure to log on regularly to Uplay.