Terraria. In the 24 hours it was out, I managed to play it for 17.9 hours (According to Steam). I'm SO addicted, and it's so fun. For those who don't know, it's a hint of Minecraft, with a hint of Castlevania.
The game opens up and what do you need to do? You need to gather materials and make a house to survive the night. We've heard that before. Cool, but later on, you build more houses and NPCs move in and inhabit them, offering different services to you! You then venture into the depths to find jewels and ore to craft new weapons and armor out of, while fighting off hoards of monsters depending on the depth.
After you're geared up it's a good time to head to a corrupted area, dive into a chasm, and start breaking open some fucked up orbs! This will either get you lucky and spawn a meteor to mine in your map, or just spawn a boss to wipe you out. Either way it's a fun ordeal.
There's tons more, and I'mma start making vidyas! The game is only $10 on Steam, just go to http://terraria.org/