Over the years I've played co-op with lots of different people. Primarily it was my brother who I played couch co-op with on the Sega Mega Drive, but eventually we both moved on to other things.
A number of years later I was finally introduced to online multiplayer thanks to the Xbox 360, and to a lesser extent the PC. It was hear that I would start to build a number of virtual friendships with people, generally facilitated by Co-Optimus.
So lets get to it, first and foremost my main co-op crew consists of handsome Co-Optimus staff member - Sam 'Samoza' Tyler. We have played/play pretty much every game with co-op campaigns: ranging from the awful, but fun, Damnation, to the truly sublime and quintessential Gears of War series. You name it, we've probably played it.
Given that we've played together for over 5 years now, we have pretty well defined roles within any game - I'm your typical run and gun/slash/stab/blow up kind-of-guy (who dies alot), while Sam is a more laid back stand in the background and snipe sort-of-chap. We're pretty much at the stage where I know exactly what he's going to do in a game and vice versa (particularly when it comes to his driving vehicles + cliffs).
Beyond Sam, I've come to know and play with quite a few people here at Co-Optimus - mainly through playing Left4Dead. These include people like Biohzrd451 and his wife, Mrxknown_JG, blakepro, TroNada, Armor Biff, Ash1138, SecretAsianManz, Deriaz and others (sorry if I've not included you). Justabaldguy is still on my "to play with" list, as it's never happened to this day…soon my friend…soon. Essentially, most of the people who lurk around Co-Optimus are apart of my larger crew.
Some of them have even joined up in my Co-Optimus co-op clan - Team Rainbow Friends - playing competitive co-op games such as Left4Dead and erm… Left4Dead. We have been known to completely obliterate other teams as a result of really working well together. It's been a while since we played, mainly because I lived in Japan for ages with no console, and then started an extremely busy new job when I got back to the UK so it's been over a year since I've played really!
Part of the problem with playing with most people I know is that they all live in America, which makes arranging an acceptable time for everyone a bit difficult… though sometimes it works out nicely on the weekends. With the upcoming DLC for L4D2, I hope to revive Team Rainbow Friends, for some colourful and joyful zombie killing.