If you pre-order Half-Life 2 through Valve's Steam, you get Counter-Strike: Source which is imo the best online FPS. There are two teams-- terrorists and counter-terrorists. The terrorists either try to hold onto hostages or plant bombs. The CT's try to defuse the bomb or rescue the hostages. You have a large selection of modern weapons, from the Glock, MP5, M4A1, M249, to the one-shot AI AWP-- and a lot more. Sadly, due to copyright lawsuits they had to change the names, but the weapons are still pretty obvious. Primarily the one thing CS is famous for is its "realistic" damage. Shooting someone 1-4 times is usually enough to guarantee a kill -- even with body armor. It's pretty realistic, much moreso than most games of it's type, but not perfectly so. Still, it's a lot of fun. The new engine update to it is great, and it runs fast even on my year-old computer.