I'm rather enjoying Champions Online (although I do have a subscription) - it's a superhero MMO mostly known for its very robust character creator.
I think that as of the latest patch, it effectively has no content off-limits to free players, and the only things you're really missing out on are the ability to colour your powers, (no biggie, usually) a good amount of cosmetic content (roughly half or so is available 'for free' to subscribers, the rest is paid-for for everyone) and the ability to make your own power combinations and builds (this one is pretty major)
Without the free-form character building system you're limited to so-called 'archetypes' that basically position themselves somewhere along the tank-DPS-support MMO trinity while emulating 'classic' superhero abilities. 'The Behemoth' and 'Glacier' are tanks, for example, being nigh-invulnerable bricks and ice-flingers respectively, while 'The Inferno' (fire-user) is a DPS archetype and 'The Radiant' is an angelic support type. There are more free and paid-for archetypes available other than those, of course, and cosmetics and powers are completely distinct. You can make a wispy behemoth or a robotic radiant, whatever.
Anything paid-for in the store you can technically grind for in-game through obtaining 'questionite' and swapping it for store credit on a sort of auction house type deal, but the exchange rates are pretty bad. Still, it's possible.
Not a perfect game by any stretch of the imagination, but I find that the superhero themes makes up for a hell of a lot.