Gunshine.net is a free to play browser based isometric strategy game with co-op play. Got all that? Good. Now head over to the official website and sign up for the beta, it takes like 3 clicks, and you'll be on your way to a Syndicate like experience.
The game offers co-op specific missions to take down bosses, as well as item trading between players. Up to four people can team up to go on a mission or you can hire your friends as mercenaries to help out. For a browser based game there's a lot of depth I saw with my limited time in the closed beta, so I'm anxious to jump back into the game now that its been opened up to the world.
To sign up for the beta, just head to SuperCell.net.