PlanetSide's the only MMOFPS I know that is truly massive. With a maximum of 133 players for each of the three sides (399 players) per continent, there's plenty of action going on. It requires a subscription to play, but when a game's this good I'm happy to pay the mere $14.99 or so per month.

Check out the website and get started here.

I loved it for the short time I played it, I dunno if its *quite* worth the monthly fee, but a great game nonetheless.