I wanted to mention this to wrap up the Battlefield posts:
B:P4F is basically a slightly upgraded Battlefield 2 game that is, as the name suggests, free to play online. It currently is in a beta stage and it's open for anyone who wants to register.
The way it works is really simple: you create your soldier which is one out of four possible classes, then you play with it, gain experience which earns you levels, which in turn lets you choose skills (for example, piloting a helicopter).
You're given the option to purchase certain things like guns, apparel and gadgets, which you have to pay for with in-game money. How do you earn money? One of two ways, and here's the catch: there's two types of money, silver and gold. Silver can be earned by playing the game and gaining levels, gold can only be purchased. And, as I'm sure you guessed it, the best weapons and equipment can only be purchased with gold.
I've been playing it and it's quite fun, although with a lot of bugs and a netcode that needs much tweaking. If you want a shooter that lets you go against plenty of other human opponents and that it's free, then you need look nowhere else.