Backpacker magazine has a great database of hiking trails that you can view with Google Maps.
From the site's home page you can select the state that you wish to view hiking trails for and you will then be shown a Google Map of suggested hikes in that state. Each individual trail is displayed on its own map, alongside the length and difficulty of the hike.
Appalachian Trail Conservancy Map
The Appalachian Trail Conservancy map has been created to help you plan a hike on the trail.
The Google Map of the trail includes information about parking locations, shelters on the trail and photographs of some of the amazing vistas on the Appalachian Trail.
Redtrails: Trails Co-Op
Retrails Trails Co-op is a compendium of over 35,000 American trails ranging from National Parks and Forests hikes to local mountain bike trails.
The individual pages for each of the trails includes a Google Map showing the route of the trail. Alongside the map are the trail's details, such as its length and elevation profile. If you zoom the map out you can also view trails that are nearby.
Tracks and Trails in the USA