Hmm well a lot of mmo's put stories in but people just don't read them. Like runescape is very story driven (just read about all the quests, not really a main story just lots of stories) but most people don't recognize that most of these actually do have a story :P
If you mean it plays like a single player rpg, like FFXI, then there really aren't any f2p ones..PSU is another one that works like that.

