Valve have some good games on their Steam service.
DC Universe online, D&D online, Star Trek online, APB reloaded are the ones i dip in and out of (dcuo is the one iplay the most...good content for free). to do well you need to buy expansions, but for the odd bit of free fun you can't go wrong really. worth checking out.
i use it when my WoW sub runs out before payday
WoW, and i think Diablo III, offer the first 20 lvls free.
also aeria games have some good free mmorpg's but nothing original, just the usual formats.