I stumbled upon this little indie gem called Minecraft. Maybe you already heard about it, if not I highly recommend it. There are two versions available here. The Classic is free to play, it's essentlially a sandbox, you can even play it in multiplayer. The alpha version costs money, but I think it totally worth it.
The game is about a world made entirely out of cubes, and you can destroy and place all sorts of cubes. Some cubes give you resources that you can craft into items that you can use to harvest cubes more efficiently.
There are no real goals only to gather cubes and build stuff. And at night, you have to survive! Zombies, skeletons, vicious spiders and fearsome creepers are out to get you!
The graphics are minimalistic at first sight, but it really fits the atmosphere of the game.