I don't like CS, but I thought Half Life was great. One of the best PC games ever. So much atmosphere, along with Thief 2.
Recently, my fav multiplayer game has been Mechwarrior 4.
Yes, I know it is a Microsoft product:eek , and I don't like Microsoft either, but it is such a great game, and it's awesome in multiplayer mode. Very fast, a good difficulty that pleases both experts and beginners, and a real sense of being on the battlefield.
Weapons are very satisfying too