It's been mentioned elsewhere, but it should probably be added to this thread:
Since March 2011, BWI passengers have an additional transit option to the DC metro area, the MTA commuter bus 201 (
schedule here). The 201 runs from the BWI to Shady Grove metro, stopping twice along the way at Park & Rides. Its BWI stops include Southwest's terminal, the international terminal, and the Amtrak station. It takes about an hour to run BWI-Shady Grove. The cost is $5, and unlike the B30, the MTA 201 uses a commuter coach with under-carriage baggage storage.
Note that the schedule is a little weird, as it's trying to provide both commuter and mid-day service. Going from Shady Grove to BWI, the bus runs hourly 4 a.m. to 5 p.m.; going from BWI to Shady Grove, the bus runs hourly 9 a.m. to 10 p.m.