Source: http://lifehacker.com/5589590/real-time-farms-is-a-user+supported-local-farmers-market-guide
The site is very community-based, with most of it being kept up by local food enthusiasts, so your city's mileage may vary. Obviously, the more you contribute (and spread the word), the more useful the site will become—you can share geotagged photos of your last visit, add or edit information about local markets and farms, and view when certain markets are open over the course of the week.
The site's fairly young, and not all cities have a ton of information—if you want to see the site's potential, plug in Ann Arbor, Michigan, the city in which the site was founded—but I searched a few other cities and found at least a small database of markets in the area. If you're already pretty into the local scene, you may already know a lot about the local markets, but you can always contribute data so your area is stocked with more information for those less seasoned. Head on over there to check it out, and let us know what you think in the comments.