The fan-made add-on that's aiming to bring multiplayer to the PC version of Just Cause 2 came out of hiding last month. And lest you think that it might recede into the background again, here's proof that the JC-MP devs have been hard at work.
The clip above is edited together from one player's session inside the beta. The impression I get watching these sequences is that you may not even need authored missions inside a big crazy sandbox like Just Cause 2. Just let a bunch of players run around parachuting, vehicle-jacking and dog-fighting until their hearts content.