The leaked version of Halo: Online, released early for testing in Russia; it is roughly similar to Halo 3. Our top scientists are currently working on reverse-engineering it so that, with what little information they have, they can enable private-server multiplayer. While you can't play multiplayer yet (as of this post), It is in the works, cant do it yet - the most you can do is load maps and run around with yourself. However, you can do plenty to mess around with in the files in the meantime as progress is made.