Minecraft
Minecraft is a vitural world made of blocks. At night monsters come out. Use the tools around you to make a house for protection. A cool thing is if you build a bed than you can sleep through the night and not get attacked. Of course you still need a house. I think it is a good choice for people who like a building challenge. Here is the website http://minecraft.net/. You will need to make a free account to play.It also has 2 types survivor and creative. In creative you have an unlimited amount of blocks.In survivor you have the blocks you destroy.