I know that the Public Market has its share of critics, but it's really starting to show some promise. Obviously, Maine has a limited growing season, but since so much local produce is currently available, the produce stands (much maligned for selling out-of-state goods most of the year) are now offering about 70% local stuff at decent prices: $2.50/lb for local organic tomatoes, $2.50 a pint for wild blueberries, .99/lb for local organic specialty potatoes. It's finally worth shopping there.
Also, the International Market (next to the wine stand, around the corner from Sugar, Commissary's baked goods counter) has amazing sandwiches the size of your head for $6 a pop. You fill out a menu slip with your choice of bread, meat, condiments, veggies. If you order pickles on your sandwich, they are dill pickles fished out of the jars in the deli case and sliced lengthwise on the slicer. Most of the bread comes from Borealis, around the corner. On Friday, the $4.50 special was a tomato sandwich, which consisted of slices of local organic tomato on your choice of bread with mayo. That's it. Brilliant. In addition, they sell local produce and a nice Sahadi-style bulk selection, including beans, dried fruits, grains, and some spices.
If you like smoked fish, Horton's offers good local stuff: salmon, trout, shrimp, scallops. They also have a respectable cheese counter. In my experience, the cheese there is in better condition than the cheese at that Commerical St. gourmet mecca, Browne Trading. Thanks to Horton's, I had a miraculous Shropshire Blue epiphany.
If you're visiting the market, lunchtime on Fridays is a great time to pick up innumerable free samples.