Planetside 2. It's free!

Planetside 2 is a free to play MMOFPS. It features 3 factions vying for control of three "continents." Each continent is a multiplayer map with many capturable bases and outposts on it. The battles in this game are much bigger than in Battlefield. Instead of the 64 players that a battlefield server maxes at, a Planetside server maxes at 2000, with 666 players per continent. I'm not sure I've ever been in a maxed population continent, but large battles regularly have 50+ players on each side. Each continent is somewhere in the neighborhood of 64 square kilometers. For comparison, Caspian Border is 2.2 square kilometers.

With so much space, vehicles are a lot more important than in Battlefield, and are not limited in number by the map. Every player can spawn a ground or air vehicle at the proper facility, only limited by a respawn timer and resource costs. This leads to huge armor columns of tanks and transports, and huge swarms of fighters and bombers.

As mentioned in the thread title the game is completely free to play. Paying real money can get you experience bonuses, unlock new weapons, and purchase cosmetic items (camo, armor, vehicle customizations). You can still unlock all weapons without paying real money, you just have to get enough experience to unlock them. Also, the different weapons are sidegrades, not upgrades, so a player willing to spend a ton of money isn't getting an advantage over one who isn't. The only real upgrades are improvements to class equipment, and those are only available through experience.

The game is available at http://planetside2.com and on Steam.

If you decide you want to play, use this invite link to register: https://recruit.soe.com/recruit/smlanding.action?iId=4N7OAOWCBNY3HNRJXKKW&gamecode=PS2
It will get you a free LMG for the Heavy Assault class and a one day experience boost.

I play in the Vanu Sovereignty (the purple ones) on the Connery Server (it's the only US West server so it should be easy to find). Let me know if you decide to play and I can show you the ropes!