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.
Montgomery Parks, part of the Maryland-National Capital Park and Planning Commission
Montgomery Parks owns and maintains the section of the CCT running from the DC line to downtown Bethesda.
Their website provides descriptions and maps of all trails in the Montgomery County Parks.
Montgomery Parks Foundation is the nonprofit fundraising adjunct to Montgomery Parks.
Montgomery County Department of Transportation owned the section of the CCT running from Bethesda to Silver Spring, now closed for Purple Line construction. Their Bike & Scooter page has many useful links, including: Bikeways and bike parking, Safety, Improvement projects, Closures, and a Bicycle Problem reporting form. MCDOT has a general Get It Fixed page for problem reporting.
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 may get security errors or timeouts. (This may be due to configuration problems around CloudFlare, which their hosting provider typepad uses.)
Washington Area Bicyclists Association (WABA)
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."
Maryland Transit Administration’s Community Advisory Teams for the Purple Line
Maryland Department of Transportation Maryland Transit Administration (MDOT MTA) has divided the 16-mile Purple Line corridor into eight geographic areas, each to be represented by a Community Advisory Team (CAT). MDOT MTA and Purple Line Transit Partners (PLTP) have committed to maintaining open dialogue with the communities surrounding the Purple Line via CATs with the objective of building a long-term relationship and working with community stakeholders to minimize impacts during construction through completion of the project. The CATs are member-driven teams with representatives appointed from neighborhood and civic associations adjacent to the alignment. There are also representatives from business associations and local governments within each CAT.
Bethesda BIKE Now Coalition
Their Facebook page says: We are a coalition advocating for safe bike infrastructure now through downtown Bethesda and to Silver Spring while the Crescent Trail is closed because of the Purple Line.
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.
Potomac Pedalers Touring Club
Potomac Pedalers is a non-profit educational, recreational and social organization for bicycling enthusiasts. They are one of the largest bicycle clubs in the United States, and the largest in the Metropolitan Washington D.C. area. They run many weekend rides, at every experience and ability level.
You may Register your bike with Montgomery County, for free
Montgomery County residents may register their bicycle and display a county decal in a visible location on the bicycle frame. This is most likely to be useful if the bike gets stolen and recovered. Registration used to be required and was made optional for non-rental bikes by Bill 2-20, that was passed by the County Council on 3/31/2020 and went into effect on 7/3/2020.
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.
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.