Garry's Mod

Indie, Casual, Simulation, Multi-player, Steam Achievements, Steam Trading Cards, Steam Cloud, Valve Anti-Cheat enabled, Single-player, Co-op, Cross-Platform Multiplayer, Steam Workshop, Includes level editor, Captions available, Includes Source SDK, Online Co-op, Online PvP, PvP, Remote Play on Tablet, LAN PvP, LAN Co-op, Tradable

Link to the game in Steam

Garry's Mod is a physics sandbox. There aren't any predefined aims or goals. We give you the tools and leave you to play.