My favorite Philadelphia fish market.  Fresh fish, great prices, bountiful selection.  Two locations, on the corner of 9th and Washington and their original location, a couple of doors down on 9th.  The counter folk know their fish and are glad to answer any questions you have on the various types of fish and the cooking thereof.  The 9th & Washington location also sells breaded seafood for frying and has a full service, albeit somewhat pricey, restaurant attached to it.