Garry's Mod
(PC)
So, Yeah, Garry's Mod. Well, most of the time when you hear about mods for Half-Life 2, you think Counter-Strike right? Well, think about an open sandbox world with no goal (For the default gametype at least,) and nearly limitless possibilitiys. That's the game out of the box. (Metaphorically speaking. It's Steam download Only) After a quick peak at Garrysmod.org (Linked at the top,) you'll start to understand that there is much more to see. The Top Downloads page has the most popular additions (.zip format only. Sorry.) and you'll find everything from guns to velociraptors, and just about everything inbetween. With a dedicated community, you'll pick up this game, and never put it down.
Screenshots:
G-man surfs the magical ballon bath-tub.
The garrys mod Spawning menu, notice the amount of tools and objects to spawn!