http://www.minecraft.netThis is a browser-based sandbox game, currently in alpha. It's point is that you can build the game world itself. For this, you have ~30 types of blocks, you can build countless pieces of each; also, you can destroy almost anything in the already existing gameworld. As one can expect, this leasds to a bunch of idiots who delete everything everyone built, but these people are banned from normal servers after about 15 seconds. Because it has a multiplayer mode with actual servers too. Also, it's currently free, but you can buy it now with 50% off and all future content free.
Tl;dr awesome free gaem, go try.