There are three attempts at a Final Fantasy tabletop RPG that I know of (not published ones - all are fan efforts, though above-average ones), plus a number of places where people have done d20-based material adapting elements of FF to D&D.
ZODIAC is a surprisingly elegant little system where each player essentially designs his or her own class, including all the weapons and such that they will eventually find throughout the campaign. It's a very different approach to character creation to say the least.
Originating right here on ENWorld,
Final Fantasy Zero is a d20-based OGL implementation, but it's gotten about as different from core 3E D&D as, say, Spycraft. It's still a work in progress, but is, or at least, should eventually be, the most comprehensive such implementation, and is certainly the highest-quality one I've seen.
And the oldest and most comprehensive is the
Returner Final Fantasy RPG. Frankly I don't like its mechanics at all, however, there are about a zillion worldbooks that may be helpful to plunder ideas from.
As I say, there are also a zillion d20 things ranging from single prestige classes for otherwise vanilla D&D to attempts to comprehensively replace the class system using FF tropes. They vary considerably in quality and there are too many for me to attempt a comprehensive list of, at least right at the moment, though I keep meaning to try someday. FF0, noted above, I think is probably the most notable one but it's by no means the only one.
