I used to be a PC gamer myself. I've played a lot of the older FPS games and some RTS games.
Half Life, Half Life 2, Counterstrike, Counterstrike Source, Team Fortress and TF2.
Call of Duty 1-3
Battlefield 1942, BF2, Battlefield 2142 and I've been considering picking of BF3
Doom, Doom 3
Brothers in Arms
I was pretty good at the height of my gaming career with Counterstrike Source, Call of Duty and in Battlefield 2142, which are the three I probably played the most and the three that were the most popular when I was deeply ingrained into it. The onslaught of cheating and hacks in Counterstrike made it impossible to stay good. Even Punkbuster didn't help in the end.
This is just a few I've played.
I also got into a few MMORPGs, like Travian ( I played 2 servers, each game takes 9 months to a year from server reset to end of game).
I played a F1 racing game as well on Grand Prix Racing Online (GPRO) for several seasons and our team reached the top 25 in the world of players from all over the planet. I used to watch the races from my mobile phone!
I haven't touched a game aside from silly flash games that you can play for 10 minutes at a time for nearly 3 years now.
I am still a member of a clan from the FPS days, but I rarely communicate with my old 'online friends', but used to hook up in servers for matches and scrims with other clans - mostly in the Battlefield game series and the Call of Duty series. My old clan was Strategic Assault Specialists (SAS), but the head of the clan took our server donations and misappropriated the funds. We split a while back and I stuck with the people who left and created Inebriated Brethren (IB). Loved the friday night frag sessions - drinks and TeamSpeak with the clan and some serious carnage. Yes, I think I've played most of them at one point or another.
Ah, the good old days... I'm 40 now as well. I will probably make a comeback when my son is old enough to start paying these games. He's just over 2 weeks old, so we've got a few more years yet.