Chris F.
Yelp
My go-to place for all things fishy. Absolutely fresh fish, knowledgeable, friendly staff, and prices that are always below, often by a dollar or even more per pound, than competitors (example: I called my wife from Whole Foods: "hey, they have a special on swordfish here, $14.99 a pound: should I get some?" Her response, "Free Range has it at $9.99". And they did.
Can't go wrong.
(UPDATE): I see from a bit of browsing that some reviewers complain that the store isn't a comfortable place to dine. That's not surprising, because it's a fish store, not a restaurant. My suggestion for those visiting Portland is to have them cook you up some lobsters (call ahead, if you like), then get back in your car and drive east (take a right) on Commercial Street and travel maybe a mile, mile-and-a-half to the Eastern Promenade. Even with summer tourist congestion it shouldn't take you more than 15 minutes, and probably less. The Promenade has plenty of parking, benches, and a fantastic view over Casco Bay. I can't think of a finer place to chow down on lobster, and between the reasonable prices at Free Range and the free view on the Promenade, you'll have the best dining in town, and you'll be far ahead of what you'd pay had you'd hit one of the restaurants on the wharfs.
(That's if it's a nice day, of course. You might want to pay up for shelter in inclement weather.)