Truely free and not having to buy a game in the first place? I only have two of those I remember playing and enjoying, though probably not your style.
Sword of the New World: Granado Espada
Apparently renamed Sword 2 since I stopped playing for some dumb reason. Interesting concept to me, has badass music as well. Is pretty much a similar world to the colonization of North America, only with fantasy aspects. You control three characters at a time, from several classes. I'm not sure how much the game has changed since I stopped playing; which, I must say, after getting a bit up there in levels it started to get repetitive. It's a Korean game, thus Korean style of mass killing of mobs. You'll most often find yourself fighting a horde of enemies and being able to get out of the battle alive, which -does- make you feel badass. Ultimately though, it does get repetive as I've mentioned, however this also might be because I played it alone. When I started, I played with people that later got busy, and I no longer had fun doing anything on my own, so it might just be my personal bias. Either way, its completely free, so its still fun to try out.
Luna Online
This is also a Korean game, and in a bit of a chibi anime style. Totally not for everyone, especially guys that might feel awkward running around in Kawaii Land. I don't really have that problem (probably because I am not a guy, but whatever), and find it enjoyable. It's world is basically a floating continent called Blue Land. The premise is the usual of fantasy type games, in which you're recruited to stop a looming evil. There are two starting races, human and elf, and three starting classes, warrior, rogue, and mage. Each of the three classes will gradually lead to other classes you can change to, giving you more specific roles or a variation of that class, or both. Gameplay is quest based, with a bit less of the mass enemy-ness that Sword of the New World features.
Also both these games are the point-and-click to move style, so if you don't like that, neither of these games are for you >.>