Garry's Mod is a nice sandbox game Where you can build every thing you want.

TerraTech is a nice game. Where you start as a little car. You need to fight other car to get bigger. You can bulid a base and fight bosses. Best game:D

Scrap Mechanic is some of the best sandbox games i have buy it is fun and you can build many nice things in it :D

GRAV is a mega cool game if you love survival games. In GRAV you need to survival, build bases, defeat monster and mush more.

Who's Your Daddy is a noice game if you are bored and dont know what you will play