Just Cause 2

A lot of people have heard about it already but there's a mod currently that turns Just Cause 2 into a 5000 person multiplayer server. It's currently in alpha I believe but it's batshit insane. Those of you who don't know about it, Just Cause 2 is a sand box action third person shooter. It contains grappling hooks, lots of guns, vehicles, and explosions. The world is absolutely massive, and gorgeous and all of it is playable in the multiplayer. It works amazingly and surprisingly well.

The server itself is only up on weekends for a day or two, and usually will skip a week or two. I can only imagine how much a 5000 player server can cost, and they're not releasing details on how much it costs them. The syncing is astonishingly accurate as well, with very very little delay. One fun thing to do with a friend is that you can use the grappling hook to attach 2 objects, vehicles, or enemies together. We tried making a kite by attaching an airplane to a car, and you can only imagine how well that worked out. Things only like to attach if both players are hooking the same object together though. That's most likely in place to prevent griefing, but that pretty much happens any way it can otherwise.

If you don't have it already, I highly suggest picking it up. It goes down to about $5 dollars during sales, and the single player itself is very fun anyway.