Minecraft is not yet available on Valve's service, but there are alternatives, such as Lay of the Land.
Lay of the Land was recently released on PC via Steam.
Southern Cross Interactive describes Lay of the Land as "an adventure fantasy sandbox world where everything is created according to the laws of physics".
Players are invited to use the environment to their advantage, as well as explore, craft, and build, gather loot, and fight.
Lay of the Land uses a voxel-based building system, and its local procedural shape system "goes far beyond simple square walls and roofs" — allowing players to create rounded structures.
The first reviews of Lay of the Land on Steam are mostly positive.