Are you bored? Are you on the internet? Do you happen to like guns? If so, Pimp My Gun is probably as good a time killer as anything. While the graphics are a little on the clipart-y side, the technical-mechanical side is pretty hardcore. Your list of parts includes upper and lower receivers, barrels, gas blocks, flash hiders, suppressors, and more. And the original version has pistols, shotguns, and an M14. Choose from a wide array of prebuilt weapons, and modify them from there, or just cobble together a nonsensical collection of parts into something vaguely resembling a firearm (see below).
It’s probably not the best use of my time. But, really. Where else can you go to make the bastard offspring of an MP5 and some kind of M4 in a couple of minutes? And it doesn’t even have a rail to mount the M203 on! If this kind of talk gets you excited… this is something you should look into.