While it's not an MMO in the purest sense of the word, Panzar does feature persistent character progression. After each match, players are granted experience and resources that can be used to level up their class of choice and craft spiffy new equipment, respectively. The game is free-to-play, so if it sounds like your cup of tea, you can head on over to the official site to give it a spin for yourself. Alternatively, click on past the cut to check out the game's official cinematic trailer.
[Thanks to SKURVEL for the tip!]