Every May I look forward to going to the Farmers' Market in at the Lincoln Square mall. They have fresh produce and meats from local growers and some other prepared foods (pies!). It's a very friendly atmosphere and it doesn't have the redundancy (Soulard in St. Louis) or pretentiousness (Clayton, MO) that other farmers markets may have.
Sadly, two of the stands I used to frequent the most are now gone. I have not seen the "russian lady" with her pirozkis and crepes, nor have I seen the pies I've grown accustomed to expecting. Alas. (If anyone knows what's happened to them, let me know)
I haven't seen any produce that I can't possibly find anywhere else (like 8 different types of nectarines at some SF farmers markets), but give it a try and I assure you that you'll find something there that'll bring you back the next week.