This game is like an old-school side scroller but it has many rpg elements and an awesome 'exploration' aspect to it. It's pretty cheap on Steam but it does have a kinda steep learning curve -- there's a npc in the game who helps you but figuring stuff out at first can take a little time. You can dig/tunnel in the ground, fly in the air, build stuff; real fun experience overall.

It even has online multiplayer!

http://www.terraria.org/

I was looking into getting this because of the steam sale, and because I wanted to see what all the Mindcraft like game craze was about. Since minecraft couldn't be bought on steam I was going to pick this one, but GTA Episodes From Liberty City went on sale for $5 and I picked that up instead. I did get Cave Story though. I'll pick this up when I make a dent in all the new games I just bought.

whens the new counterstrike come out, i will be getting this!