I found this game a few months ago and got hooked. It's in alpha right now and you can get it for half the price at $13. When it goes into beta the price will go up.
Basically the game is legos, where you build shit. There are 2 modes as of right now, survival and creative.
In survival mode you basically just try to stay alive with no real objectives yet. You build a fort and fend off monsters during the night. Notch, the creator of the game, has been developing a test survival mode called infdev which involves playing in infinite worlds. Currently this mode is only single-player, but Notch plans to add multi-player soon to it.
In creative mode you're given infinite building blocks and you build whatever floats your boat. This mode has both single-player and multi-player, which is cool because it's way more fun to be building with other people.
Notch plans to add other gamemodes in the future aswell.It's made in Java and can be played in a browser
Here are some pictures of it