In the ever-expanding desire to take sandbox games to their ultimate conclusion, Minecraft and games like it allow you to literally do whatever you want. Think of an actual sandbox where you have the tools available to not only mold the sand into whatever you desire, but also to make the sand itself. Now toss in a little Call of Duty and you have Guncraft. Sure it looks like the world's most primitive shooter, but it also lets you build your own cover on the fly.
Furthermore, if you happen to find yourself at Boston's PAX East this year (which starts Tomorrow, so you better start packing), and you happen to stop by the Reverb Publishing booth (#1048) to play Guncraft, you can score yourself a sweet exclusive character skin.
Finally, Guncraft has been submitted to Valve's Steam Greenlight project, so if you want to see Guncraft on Steam in the future, you can go here to vote for it.