If you like exploration, discovery, and finding things out, get Terraria!
Only $10 on Steam. It's a bit like a 2D version of Minecraft with a huge (randomized) map. It has a bigger emphasis on exploration and figuring out how to get strong enough to defeat the bosses (if you can figure out how to find them first)
I wish more games would give you so much freedom to alter the world. My project to drain the ocean into the lava layer is coming along nicely.