Garry's Mod is a FPS multiplayer/singleplayer Sandbox game, that is VERY customizable and costs 10$ OR buy the Counter Strike Source + Garry's Mod for 24.99. You can create anything your mind comes to, from making weapons with coding and modeling, from making that one contraption nobody made. It supports Steam Workshop, has loads of addons in store in the Steam Workshop or on this site:
http://www.garrysmod.org
Beware, some addons may not work with Garry's Mod 13.
Here's a old video of a time bomb made using Wire + Advanced Duplicator (To use this you must have a SVN client, I use TortoiseSVN). Download link Sounds required for addons, put them in the sounds folder: 1 2 (DL lasthuman_test.mp3.bz2)
Anyways, discuss your latest creation, post amazing addons, etc