Planetside 2 is an free MMOFPS game developed by Sony Online Entertainment, creators of Everquest and DC Universe Online.
To describe it in one sentence, it is an FPS game with vehicles and planes(Think BF3) with a stage(Day/Night cycles on a 8Km by 8Km continent) that support up to 2000 players at the same time fighting for their fraction(3).
Meaning that it's like an mmo that continues to run even after you log off mixed with the elements of an FPS.
Trailer:
E3 stream with commentary by Totalbiscuit and Margaret Krohn:
Obviously it's better to team up than solo in this game, so I'm trying to brainwash convince people to join the Vanu fraction.