The Just Cause 2 multiplayer mod has been around for a while on the PC, but it's going to be getting an official release on Steam. This means you will be able to download the mod and server right from Steam, and you'll be able to join in on your friend's games.
There will be a final beta test on December 7th, and beta keys are available. The mod should be released by the end of the year, so even if you don't get in on the beta you'll still be able to play Just Cause 2 with your friends soon. Hopefully it hits a good price on a Steam sale.