I"m in Montgomery County and with their archaic beer laws it's tough to find a decent selection but here is the best one I've found for total selection:
Olney Beer and Fine Wine Olney Beer&Fine Wine 301.774.7593 Great selection and they will let you mix and match six's with a 10% discount. They also have a beer of the month club, special tasting events and a very knowlegable staff. If someone can find a better selection in MoCo please let me know.
I also get up to Ye Olde Spirit Shoppe in Frederick, +1 for them.
If your in the Baltimore Area you can't beat Corridor Wine and Beer
(3321 Corridor Marketplace
Laurel, MD 20724)
It's more of a supermarket of beer and wine then a beer store... if you can't find it there they will order no problem, if they can't get it... you can't have it in Maryland lol.
If you're ever in the Hagerstown area, you've gotta check out the Wooden Keg. It's a small place, but they are expanding and have a great selection. It was a total shocker to walk in and find such a wide variety in Hagerstown of all places.... :-)
Also, in the Frederick Area, Ye Old Spirit Shop on 7th street is surprisingly amazing, same as Wooden Keg, you don't really think that they would have a good selection from outside but I would say a full 3rd of the store is micros and hard to finds. Always find what I'm looking for there.
I'll concur with the recommendations for the Perfect Pour, Stateline and The Village Pump. That's where I do the majority of my shopping for beer. The Perfect Pour has beer and wine tastings on Fridays and Saturdays in the evening.