Free Tours by Foot offers walking tours of Washington, DC on a name-your-own-price model. The company does offer other styles of guided tours by bike or bus and all cost what you thought the tour was worth.
The signature and most popular tour is the two-hour National Mall tour or you can zero in on specific interests on the Lincoln assassination walking tour, the Capitol Hill and Library of Congress tour for the political-minded, sample some food on the Eastern Market dessert tour or take a chilling walk around the city with the Ghosts of Georgetown or White House ghosts tour. The company also arranges private tours for business groups or larger families.
Reservations are required for these free tours and can be easily booked online. Most tours are about a mile or two in length and the tours last approximately two hours.