The Just Cause 2 multiplayer mod took the world by storm a couple years ago, somehow allowing for thousands of concurrent players in a single game. It’s utter madness, and now it’s gotten a big bump up from version 0.1.4b to version 0.2.
Version 0.2 brings a host of cool changes to the mod, from basic stuff like an overhauled main menu and better reliability, to way crazier stuff like Steam achievements and support for custom scripts to allow things like wingsuits and submarines into multiplayer. It’s a pretty significant update, and it’s worth checking out the entire breakdown of changes for yourself.
You can grab the new version of the multiplayer mod from Steam for free, but keep in mind that you’ll need Just Cause 2 proper to play it.