Thanks to WMATA’s recent release of transit information for software developers, DC is now part of Walk Score’s “Transit Time” maps. They show the spots you can reach in 15, 30 and 45 minutes by transit and walking. Update: it looks like this only includes Metrorail so far.
Go here, then zoom way out to pan the map over to the Washington, DC area. They haven’t updated the dropdown menu yet, but the mapping application works.