Yeah, counterstrike is (somehow
) the most popular still. There's this other, really awesome half life mod called
Day of Defeat, which is based on WW2 and is getting very popular now. It's the second most popular half life mod. Team Fortress is still going fairly strong as well.
Quake 3 and UT are still played online a lot, but not as much as those above AFAIK.
I'm just downloading this new free FPS called 'American Army' from fileplanet. It's meant to be pretty good - it's officially endorsed and designed by the US army and is meant to be really realistic. We'll see...