This list has the most recently added sites at the top, so it's not in alphabetical order,
They also offer a "Bicycle Route Finder" service, although their GIS database of bike paths seems weak as of November 2018.
This site for "cycling advocacy in the nations capital" has very extensive and timely coverage of many bicycling topics in the greater Washington DC area. This links to their reporting on the Capital Crescent Trail, newest at the top, scroll down for older.
Note that you MUST use the full "https://www.thewashcycle.com" URL. If you leave off the "https://" or "www.", you will get security errors or timeouts. (This may be due to configuration problems around CloudFlare, which their hosting provider typepad uses.)
Capital Trails Coalition (CTC) is a cross-sector collaborative effort to build a world-class trail network in the Metropolitan Washington DC region. Their emphasis is on filling gaps in the existing trail network.
This is a "work in progress", a new website still under development. From one of the authors,
"The website collects and shares information on how to do the great long-distance car-free bike paths in North America -- where the paths are, narratives on their settings in nature and history, information on lodging and camping, restaurants and food, ice cream and beer, bike shops and shuttle services -- basically, what the touring cyclist would want to know to plan a trip that could be across the state or across the continent."
Worth a visit if only to see their page on the C&O Towpath.
Rails-to-Trails Conservancy is a nonprofit organization started in 1986, dedicated to enriching America's communities and countryside by creating a nationwide network of public trails from former rail lines and connecting corridors. The CCT is an early such trail.
An organization dedicated to promoting a national network of trails. This website contains much information on building and maintaining all kinds of trails.
The largest bicycle association and resource in the DC Metro area. Their website has much useful information on bike routes throughout the area.
Lists many rides and trails in the DC area, with good maps and ratings for terrain, traffic and scenery.
This Parks Division official website provides a description and map of all trails in the Montgomery County Parks.
Information and a detailed map of the W&OD Trail in Northern Virginia.
The site has info on C&O Canal history, current conditions and volunteer restoration activities, There is also info on joining the Association to help out.