Quake 3: A True Classic That Never Dies
I started to play Quake 3 soon after its source code was licensed under the GPL. I must admit, I didn't play it from the beginning, and I'm at best a good player, but I always enjoyed playing online some classic mode.
Quake 3 was released in December 1999, and ever since, it is one of the most played online FPS games. It's actually one of the most played FPS games from all time. It is very alert and benefits from lots of maps and mods, making it probably the most played games ever, together with Unreal Tournament.
The servers are not that crowded, at least not as they used to be, but, considering it's almost 9 years old and still active, it's one of the games which will never die, which will always have both new and older, nostalgic players.