SteamWorld Dig is a charming platformer with some neat procedural generation tricks. Basically, your little playable cowboy robot must dig downwards through the ground in order to unearth stuff, with the whole affair feeling like a Minecraft / Metroidvania hybrid. I lost countless hours to this diabolical game, and now you can too: it's currently free on Origin.
The offer is available "for a limited time only", but since you know about it right now, go forth and secure it. It's good timing, because SteamWorld Dig 2 comes out later this month, and while it undoubtedly adds a bunch of new features to the format, playing its predecessor is a good way to establish whether the new game is worth taking a punt on.
Here's the sequel's trailer, for your interest.