It's Saints Row: possibly one of the most interesting series ever made - and co-op just makes it twice as fun. At least.

It's never been so entertaining to see Ghandi declaring war every 5 turns. Good if you like RTS-esque games.

I've had great banter on CS:S, and I hope I always will. Great with friends.

It's Counter-Strike - it couldn't not be on here.

ETS2 is great, but the multiplayer mod for it is entertaining. Skype/TS some buddies across the ocean, and hit the road together. Just don't crash..

Not HL2 itself - but 'Synergy', the HL2 mod. You know how there's one Gordon Freeman? Right, well try up to 8 of them. And you said 3's a crowd.

There's COD, Battlefield, ARMA etc. And then something like Insurgency appears - no HUD, no mini-map. There's PvP, but Co-op is strong on this.

Zombie-survival games are flooding the market, but NMRIH is different from the norm. It's free, it's fun, and development is active, even after 7 years.

The first laid the foundations for Payday 2 - an extremely solid co-op heist sim. Lots of action, bullets, and drill-jamming. Co-op highly recommended.

One for the nostalgia fans: the multiplayer's down after the GameSpy shutdown, but GameRanger works a treat for those looking to get online again.