Uberstrike

Check out this game...

I've been playing it for a couple of weeks... it's very cool. Browser based, also playable via Facebook. There's also a Mac client. Maybe a PC one as well...dunno.

Free to join up and play...no spamming unlike Farmville. While there is a micropayment model for weapon upgrades, you also accumulate points for killing and just for showing up that you can use to rent or buy weapon upgrades for various amounts of time (days, weeks, months.)

You can easily get by with the base weapons because there's a lot of pick-ups in game, and you also can loot the bodies of your slain enemies.

The game client version has an undeniable advantage FPS-wise over the browser version (100+ fps vs 50-60fps for me.) Anyway, check it out. My tag is "Datasquid" if anyone wants to hit the servers for awhile.