There was a fair amount of press for Wilson's Farm in the Irish bread thread. For those in the neighborhood, I also wanted to suggest you check out Busa Farm, especially in the summer (this time of year selection can still be quite good, but is variable).
They also have a CSA and their own produce is grown w/o pesticides. They carry good quality poultry and eggs, as well as some . One of the local restaurants (I think Scutra) buys produce there in season. A fish company comes in on Tuesdays, as does Hi Rise bakery. Its almost never crowded and their prices are good (bit more in recent years). In the spring they have a pretty ample selection of starts.
Wilson's is great, but I just prefer the low-key experience of Busa. If I go to Wilson's, I park across the street or in the staff lot!
Doran's is another place worth checking out if you are in Lexington. Their bread was never as fresh as the other too and prices a bit high, but convenient and excellent for flowers. Wagon Wheel I have never paid a whole of attention too.
There also is a great place for starts in Burlington on 3A just by the 99 and (I think) right near the Fish House and Grill that someone mentioned.