Garry's Mod is a Half-Life 2 toolset that lets you treat Valve's Source engine as a kind of virtual sandbox. People have been stacking the good crate (the crutch of the lazy FPS level designer) into pixel art creations.
Mario | Shammus (Metroid) | Sonic | SpongeBob
I just reloaded Steam to try DoD. Maybe I'll give Garry's Mod a go.