At The Local Beet, we've just launched a new farmer's market locator tool that I think many LTHers will find useful. The unique feature in our tool is that markets that appear on the map are color-coded based on how soon they are open (to your selected date). So, if you know that you have a meeting in Libertyville a week from Tuesday and you'll have some time to kill in the area, you can enter the zip code and date and see exactly what markets will be open near that location at that time.
You can check it out by entering a zip code in the top right corner of our home page: http://www.thelocalbeet.com
Also, we've put together a lot of data from a number of sources, including consulting from Robin Schirmer, a local farmer's market coordinator. We believe that, while it's still growing, we have the largest and most accurate set of market data available in the area. Currently, it contains IL markets only (and a few are still being tweaked) but we plan on adding southern Wisconsin and NW Indiana in the coming weeks.
A special nod goes to LTHForum Sysadmin germuska for bringing his technical skills to this project.
I hope you find it useful and I'd love to hear any comments or questions. Like everything else on the Internet, it's a work in progress. I'm making frequent little changes and tweaks to it to make it more usable.