What, you're not playing this already? It's been going for four-and-a-half years, has had over three-HUNDRED updates, and still going strong. Seriously. Give it a try. It's FREE!

Ah, it's that time of year again. When Steam asks us to write a game recomendation for the potential of free stuff. Anyway, I'll stop being meta now.
Yes, it's good. If liked the first one, but want a bit more story, play the expertly-written single-player. If you want more blisteringly hard puzzles, grab a friend and play the multiplayer.

So yeah. Terraria. If you haven't heard about it yet, you've probably had your head under a rock or something. The "2D Minecraft" comparison will come up, of course, but that doesn't do it justice. If Minecraft is a sandbox game that happens to have some action elements, then Terraria is more an action game that happens to have sandbox elements. Mind you, it's still a really ~good~ sandbox, if that's what you're looking for, but the real fun of this game is in exploring the world and making it yours.