I played CS religiously from versions 6.0 to 7.5 beta. Then I got sick of the stuff they kept changing. But while I played, my M249 was a force to be feared. And the guns are quite lifelike, and are in fact all real.
I also used to play Quakeworld TF, and in my opinion was the greatest online game of all time. For some reason TFC just doesn't have that little somethng that made TF so addicting.
These days however I play Mechwarrior 4 and its addons, and Diablo 2.
And for the FPS fan, try out Clive Barker's Undying if you havn't all ready.