Private transportation from Washington, DC, airports: SuperShuttle, a company offering group shuttles and private transportations to and from Washington, DC, airports to the conference venue or hotels is offering a discount to Wikimania 2012 attendees. The discount can be accessed here.
Washington Metro: A convenient and fairly inexpensive way to get to the heart of DC. Fare ranges from $1.70 to $5.75, depending on the time of day and distance traveled.
Metrobus: A bus system with stops throughout the Washington Metropolitan Area. Fare varies based on the bus route, $1.80 with cash and slightly less if you use a SmarTrip card (which must be purchased in advance, on-line). Express buses to the airport cost $6.00.
DC Circulator: A bus which connects various popular neighborhoods (Georgetown, Chinatown, Adams Morgan, ...), the venue (GWU), the hostel, and Union Station. Fare is $1.00.
Capital Bikeshare: Capital Bikeshare is a bicycle sharing system with service in Washington, D.C. and Arlington County, Virginia. A one-day membership is $5.00, a five-day membership is $15.00, and usage fees range based on time per use.
Taxis are readily available and meters are used for fares. The starting rate is $3.00 for the first 1/6 mile, and $0.25 per additional 1/6 mile. The maximum fare is $19.00. Numbers for two taxi services are (202) 387-6200 and (202) 829-4222. (Taxi Fare calculator)
Walking is another good option for getting many places, as Washington is a very walkable city.