A vote for Dulles airport is that Metrobus started a 1 trip per hour between Dulles and Reston/Rosyln/L'Enfant Plaza almost the entire day. And it is only $1.10, takes about 45-50 minutes to DC. I heard but can't confirm that this service has been expanded to 15 minute frequency during part of the day. Here's a link to the current schedule, bus 5A.
http://www.wmata.com/timetables/VAsched.htm
Hope this helps...