Syntensity is an open source platform for 3D multiplayer games that lets everybody both play and mod everything. In other words, you can mod any of the games on Syntensity, and play the new version with other people.
You can create and run games on Syntensity for free; in return, you let people play and mod your games on Syntensity. Alternatively, you can use just the engine, separately from Syntensity, and license your games however you want - for example, you can make proprietary games and sell them. (The engine's license is the AGPL, and works like Blender's GPL: The license only applies to the engine itself, not to games you create with it.)
Windows and Ubuntu binaries are in the downloads section of this engine here on Mod DB. For compiling from source, please see here.