Cumberland Trail Conference
Both of us have worked on trail building with these folks. Their website (which includes an interactive map) is a great place to start planning a trip to any part of the trail. However, their organization no longer exists, so it may be just a matter of time before their website disappears.
Friends of the Cumberland Trail
A great site to go for information about everything from music, to butterflies, to volunteer opportunities.
Cumberland Trail Volunteers
This is a group of informal volunteers. They have maps and gps track-logs of the trail. If you want to avoid getting lost, this is the site for you.
This site features an interactive map of the whole Cumberland Plateau region. Click on the icons to learn more about different attractions and plan an itinerary. Alliance for the Cumberlands runs the site.
Great Eastern Trail
The Cumberland Trail will eventually be part of the much larger Great Eastern Trail which will run from Alabama to New York. This site describes the work being done on it.