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.
Syntensity 1.1.4 for Windows. See below for installation instructions.
sweet! looks great, downloading now ^___^
Hmm, I just noticed that the screenshot is from a game that isn't in the Syntensity Lobby yet. While you can play a preview version of it (search for "GK Swarm" in the list of activities on the website), it still needs some more polish.
(If you need help running the preview version, people are usually on IRC, #syntensity on FreeNode, that can explain stuff).
To be more specific about the status, what you see in the screenshot is present and works, but we are adding more elements (doors, traps), and also we will improve the performance some more before we put it in the Syntensity Lobby.