Jeshibu's also mentioned Arma 3, which I'd also be up for; I have more missions for A2/Operation Arrowhead, but there are a bunch of things that A3 does better, not least the integration of mods. It's currently 50% off on Steam, so if anyone thinks they'd be up for it, now might be a good time.
Jesh's also mentioned SWAT 4, which with the expansion allows for 8-player coop. I've not played a lot of it, but what I played was great. Again, might be one to check out, though I'm not sure it's legally available. (He's also mentioned some other options at the Steam discussion linked above.)