Hexic HD has been free on https://Xbox.com since its initial release. It was one of the early BC titles to be posted (I still have the BC "test" version on my account along w/the retail version).

Anyone with an Xbox account can still grab it from the current storefront:
https://www.xbox.com/en-US/games/store/hexic-hd/BXHL9BJH9XKC/0001