Clipper City!
The tour is $5 and you get a souvenir pint glass and 6 tickets for sample tasting (their "samples" are more like half-pints). They have a couple of brands (Clipper City, Heavy Seas, Oxford). The Clipper City brews are lighter ales (a gold ale, etc.), the Heavy Seas are heavier, bigger beers (really good IPA, saison, barleywine, etc), and Oxford are organic (a raspberry wheat and I think a red ale).
The brewery is located just below the Baltimore beltway (southeast side of the beltway). More often than not, the owner gives the tour and he is a fun guy. I HIGHLY recommend the tour.