I recommend Champions Online, a superhero MMORPG. While there are a number of perks for subscribers, and a number of items that can be purchased from the internal "C-store", it is entirely possible to play a very enjoyable game, all the way to top level, for completely free. The basic Silver account gives you two character slots, eight predefined Archetypes (varieties of superhero) to choose from, and a pretty fair array of costume parts to decorate your character (and unlike some games, if you get better gear later, it doesn't change your appearance). There are three completely optional Adventure Packs you won't be able to access, although with the latest addition, the Comic Series "Aftershock", they're getting away from that paradigm; there are nine more Archetypes and the ability to create Freeform characters that cost extra (in fact, the Freeform is one of the perks reserved for Gold-level subscribers); and of course there are even more costume bits available through the C-store.
I do have a Gold account; however, due to economic fluctuations, I'm going to have to let that account go inactive for a bit, so rather than fuss about with converting some characters back to Silver again, I've created a separate Silver account. To give you an idea of the possibilities for characters, here are my Gold heroes, and these are my Silvers.
_________________
Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.