The best online PC games are ... well as you know CS rocks ...
others...
1.All the battlefield series ... I myself play bf 1942 and it's mod Forgotten Honor which is a derivative of the Forgotten Hope mod
including Battlefield Bad Company 2 and Bad Company 2 Vietnam as well
2.Call of Duty 2 and 4 are real good but like all online games are a little blighted by cheaters.
3.Brink ... though it's slightly different playstyle ... (my son plays this ((a counterstrike freak)) and thinks it's pretty good)
4.All Points Bulletin Beta ... superb !
5.Battlefield 3 .... It's going to ROCK big time and is using a new engine
A must buy for first team shooters
Check this BF 3 trailer (way better than the teaser) :
http://www.youtube.com/watch?v=2zw8SmsovJc