Epic Games wants you to pay attention to its new store, and it's not playing. As of right now, and until December 25, the outstanding underwater exploration sim (and horror game) Subnautica is free to take, and keep. This was previously announced shortly after the Epic Games Store launched.
Subnautica is really great: Philippa called it her favorite game of the last five years in her 89/100 review, and Steven explained the importance of water bottles in this all-too-true tale of spinning small troubles into full-blown catastrophes. It was enough to convince me to try it, even though survival games aren't really my thing, and they were so right—it's one of the best things I've played in years.
Not much more to it: Go to the Subnautica page on the Epic Games Store, click the "free" button, and go for a swim. You won't regret it.