Guacamelee. Hadn't even heard of this game, until it became a freebie on Games With Gold for XBox One. It's a colorful, fun game with clever combat and puzzle platform elements, and a neat luchador aesthetic. Some of the platforming is fiendishly, hair-tearingly difficult, but the designers cannily put the hardest stuff in the bonus and optional sections, so the main story remains accessible.
Gotham City Impostors. Another Games With Gold title I got for free. I had actually played the demo for this when it came out, at the encouragement of a friend who had it. It wasn't bad, but it was a little rough around the edges, I had a lot of other things to play, and as a small-scale multiplayer-only title, I foresaw deserted servers within a few months.
Still, the concept was clever (you're basically playing as the goofball hockey-pad vigilantes from The Dark Knight, with a similar faction for the Joker), so when it became a free title, I thought, "Why not?" And now I regret not picking it up earlier - it's a fast-paced CoD clone with loads of humor and surprisingly deep gameplay elements. The gadgets are wacky fun, the level of customization is great, and it's one of the few competitive FPS titles I can play badly at and not tilt out, because it's just so engagingly silly. And while the servers aren't exactly full to bursting, there are still matches to be had.