Do you mean farming as in FarmVille style farming, or a game where you can farm things?
If its the first one then the only games I can think of are SimFarm and FarmVille. SimFarm is pretty fun, but you can't make any money off of it, and as for farmville I am not sure if you can make money off of it, but it is a social game so there is a chance if you find the right niche in the market.
For the second option, I would suggest SWTOR or RIFT as those are the only two games other than Runescape and WoW for farming things. You can farm anything in any game, but to make a profit you need to play a major game such as the ones I listed above.