I think I've heard of some other shops that might be closer, but they don't have websites so I can't get a feel for their selection, so I just keep going back to Maryland Homebrew.
The only other homebrew shop i know of is Annapolis Homebrew.
It's off of Ritchie Highway just north of the city. I do like it, I think their prices are fair and they are in the process of expanding right now. Apparently their getting a walk in fridge for yeast and hops.
I go to the flying barrel in Frederick. Its in historic downtown. The prices and stock are good.
They have "brew on-site", as there are 6 30 gallon natural gas kettles and instructors for the newbies.
It's worth the trip for the non-locals (especially this time of year) as there is a lot going on downtown and there will alway be people brewing on-site.
The Thirsty Brewer in northern Baltimore County (Baldwin, right at the south end of MD 165) is a great place to go. The owner, Tom, is very knowledgable, friendly and very nice to deal with.