I'll start.
L4D Launch.
I remember when the beta for L4D started. It was a couple weeks before the launch.
I was really excited about this. For me it was this whole new concept of a game, never had I touched a game like this before. At the time I was only 11 years old (2008). It looked like so much fun, and I really wanted it. lockwyn played with a couple of his IRL friends during beta and I would also try it out with them while playing on his computer. The only map that you could play was No Mercy. It wasn't the full one either, only map 1 and a shortened map 2.
Ultimately, I got the game as an early birthday present. (Couple of days before release)
There was a bunch of UKCS'ers who gathered a group of people for versus mode (4v4). I remember
Alias being one of the drive-forces for this. He'd gather a bunch of people for mass-chaos in L4D and we'd always have a blast.
I don't remember all the people that joined in on this chaos, if you do remember, do type it! It's one of my favorite moments in gaming.