For anyone that enjoyed the massive battles of Planetside back in it's glory days, a game developed in China (as Welkin 4591) has been "loosely" translated to English and released as Mars War! (The application icon still reads 4591 actually.)
Anyway, it's free-to-play, only a 300 MB download, and addictively fun combined arms fighting (infantry + armor + air). You earn crystals for playing and completing daily missions, and those crystals pay for a 3-day license to use your prefered weapons. Just playing for a couple hours last night I got 2000 crystals. My prefered gun is 1200/3 days; so playing regularly means you'll have access to the weapons/vehicles you prefer. I think ~$12 gets you a 30-day license for basically everything. It just launched yesterday (Jan 10) so now's the chance to get in at ground level.
It feels a lot like Planetside, the battles are brutal and fast, and there's a lot of depth to the strategy (once people stop zerging the spawn room and read a little). It's not going to kill off another MMO, but it's definitely going to be worth the price! I've got a HD stream (720p) of me playing around last night on a playback loop, as well as on demand. It gets more exciting as I figure out what I'm doing more and more. I'm not a great FPSer though.