Many people are familiar with the MMORPG, but there is the sometimes overlooked MMOFPS. Both genres have some degree of overlap- a lot of times the online FPS game will have some vestiges of ‘leveling’, in the form of gaining access to newer/more equipment. I always had a soft spot for the MMOFPS, because it allowed a player to fight on a scale larger than most conventional FPS battles. Instead of a dozen people vs a dozen people, it could be hundreds of people vs hundreds of people. There are two I can think of off the top of my head-
WWII Online- Came out a long time ago, graphics are very dated looking. This game was supposed to be very realistic, fought in a 1/3 scale of Europe. Due to the realism, there was a rather steep learning curve (I haven’t played it, but read forums, comments about the game).
Planetside- My best friend and I played this for a year. I really enjoyed it, it was a futuristic game fought over entire continents. During the right day of the week, there could be massive pitched battles, and even observable ‘fronts’. My friend and I crewed a tank, and had a lot of fun taking part in massive ground campaigns. Defending stricken bases was fun too, lots of ‘Alamo’ type moments. This game was fun to experient with tactics, like organizing saboteur squads and flack traps, or being a Lone Wolf and running guerilla tactics at 3 AM 
Any other significant examples? I really enjoyed Planetside, but for as much as it had to offer, I was always left craving more (bigger battles, more vehicles etc)