Martin Johnson of Baltimore notes an everyday option for getting to and from Washington without a car that might not be familiar to most readers.
Bolt Bus offers seven buses a day from Penn Station to the Greenbelt Metro Station each day at a cost of $15 on weekdays and $16 on weekends. Once you're in Greenbelt, you can reach almost anywhere you'd want to go in the Washington area via subway and Metrobus.
Whether this is a good deal depends on how you value your time and how easy it is to catch the bus. The Bolt Bus ride takes 45 minutes. Greyhound takes 55 minutes to deliver you to downtown D.C. at a prevailing cost of $13.50 nonrefundable, $17.50 refundable. It has many more trips but a poorly located station in an industrial area south of the stadiums.
The most economical 7-day-a-week option is still the combination of the light rail ($1.60) to BWI and the B-30 bus ($3) to Greenbelt Metro. That trip (measured from Mount Royal station to Greenbelt) can take anywhere from an hour and a quarter (roughly) to an hour and three-quarters depending on whether you catch the train that's synchronized with the B30. Checking the schedules is well worth the time.
Of course, the MARC train remains the best way to get between the two cities on weekdays -- except during periodic service meltdowns. It's a wise MARC rider who prints out the Bolt Bus, Greyhound, light rail and B30 schedules and keeps copies in a purse or briefcase. You never know.