The open beta for Just Cause 2 multiplayer has released today! It's going to be absolute, glorious, chaos.
You need to have Just Cause 2 on Steam to play, and then you can just
grab the mod and let loose. If you need me I'll be on RedCat's Freeroam Server, shooting down helicopters with a bazooka.
Some helpful commands, for reference:
/localchat: toggle chat to only display players nearby - this is the first thing you do.
/f join [factionname]: join faction, or create one
/f list: list factions
/f goto [playername]: teleports to a player in your faction
/w [playername]: PM someone
/fp: toggle first person perspective
You can find the rest in the F5 help menu.
Edit:
The game is even on sale until the 19th!