This list is ordered by a guess at usefulness to CCT users and advocates, not alphabetic or time order:
Facebook: Capital Crescent Trail Users Group - https://www.facebook.com/groups/capitalcrescenttrail/
This Group gets lots of CCT news and alerts posted. The best place to post and check for trail conditions, lost items, etc.
Facebook: Capital Crescent Trail page - https://www.facebook.com/pages/Capital-Crescent-Trail/112684482079366
This Page gets mostly CCT trip/event reports and pictures posted. Similar name, but very different from the Group above.
This Parks Division website provides a description and map of all trails in the Montgomery County Parks.
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, as of April 2019. If you leave off "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.)
Washington Area Bicyclists Association
The largest bicycle advocacy group and resource in the DC area. Their website includes useful information on bike routes throughout the area.
Capital Trails Coalition (CTC) is a cross-sector collaborative effort to build a world-class trail network in the Metropolitan Washington DC region. CCCT is a member. A major emphasis is on filling gaps in the existing trail network.
Purple Line Now
Their web site says: "Purple Line NOW! is a coalition of business, labor, environment, neighborhood, and civic organizations that works with local, state, and federal government officials in pursuit of our mission to build the Purple Line. We say "Finish The Trail/Build Light Rail" because building the Purple Line and finishing the Capital Crescent Trail go hand in hand."
Rails to Trails Conservancy
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 among the first such trails.
Commuter Connections has surprisingly extensive Bicycling pages, oriented towards commuting. They offer a "Bicycle Route Finder" service, although their GIS database of bike paths was weak as of November 2018.
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.
An organization dedicated to promoting a national network of trails. This website contains much information on building and maintaining all kinds of trails.
Lists many rides and trails in the DC area, with good maps and ratings for terrain, traffic and scenery.
Friends of the W&OD Trail
Information and a detailed map of the W&OD Trail in Northern Virginia, another major DC area rail-trail.
The C&O Canal Association
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.