Here are the ones that do distribution that I know of off the top of my head that are headquarted around here:
Capital City Brewing in the city.
Old Dominion in No. Va.
Hook and Ladder is a contract brewery in Silver Spring.
Fordham's in Annapolis.
Clipper City in Baltimore.
Wild Goose in Frederick.
I know there's another contract brewer in Bethesda, but don't remember the name.
There are also the following brew pubs thta brew on premise:
Ellicott Mills in Ellicott City
Some Gorden Biersch's and Rock Bottom's are around here.
Dogfish Head in Gaithersburg (I'm not sure if they brew on premise).