The best things in life are free

There is an enormous list of free to play/try MMOGs at mmolist. Although not an MMOG by any stretch of the imagination, I highly recommend Allegiance. Microsoft, of all companies, released it for free (including the source code) awhile back. There is a steep learning curve, however, and it may take some time getting used to all of the gameplay elements. With games like BF2, however, I think that a lot of the novel elements that were in place when Allegiance came out are more acceptable/known to the gamer community.

Groundbreaking gameplay elements included a commander who controls the technology tree, miners who help mine resources for your side, and various combatants who fly everything from small nimble fighters to larger attack craft (wikipedia entry here).