Garry's Mod is a mod of Half-Life 2 created by
Facepunch Studios. It was originally a physics-sandbox game, but since has been riddled with amazing user-created gamemodes! Want to make a spaceship? Cool, you can! House? Base? Airplane, aliens, ragdolls, props, rubber balls, cars, half-life NPCs, workings pianos, awesome maps, this game has EVERYTHING.

Garry's Mod has an awesome add-on system. There are many things created by the community in the
steam workshop, including vehicles, maps, weapons, tools, saves, gamemodes, NPCs, effects, and props! There is even a
bedroom map made by our very own Marcem. How cool is that?
RequirementsPCOS: Windows® Vista/XP/2000
Processor: 1.7 GHz Processor
Memory: 512MB RAM
Graphics: DirectX® 8.1 level Graphics Card (Requires support for SSE)
Hard Drive: 150 MB
Other Requirements: Internet Connection
MACOS X version Leopard 10.5.8 or Snow Leopard 10.6.3
1GB RAM
NVIDIA GeForce 8 or higher, ATI X1600 or higher, or Intel HD 3000 or higher
Mouse and Keyboard
Internet Connection
Also, here's a fun tool I made: fading doors. Check it out if you'd like!