Update: The link below for Project Stream sign-ups wasn't working for some people, even if they're in the US. That should be fixed now, but if that happens again for you, I suggest Googling the project separately and finding a link that way.
Also note that signing up for Project Stream doesn't guarantee you'll get to test it—Google will review your application first (although the promise of freebies suggests they're looking to get as many testers as possible).
You can only sign up to the project in the US, and you have to be 18 or older. Project Stream, currently in its "technical test" stage, will stream the game to your laptop or desktop via Chrome, which you'll need to install first, and you'll also need to have a linked Uplay account to get your free copy. A fair list of requirements, then—but getting Ubisoft's excellent action RPG for free will surely be worth it.
All your game saves and in-game items from the Project Stream version will transfer over to your free copy. You have until January 15 to sign up for testing.
Anyone that test Project Stream, no matter for how long, will also receive 1,000 Helix credits, equivalent to $10 on the Uplay store, Ubisoft said in a blog post.