Whether you're hiking all 2000+ miles of the AT, or just want some insight into one of the world's most famous hiking trails, this is the place to come. From the ATC Headquarters, the care of the entire trail is coordinated.
They have a small, but informative display about the AT, the highlight of which is a large 3D topo map of the entire trail from Maine to Georgia. There is a hiker lounge is the back where you can flip through volumes of people who are trying to hike the whole thing, either all at once, or in pieces. If you backpack in, you even have the option of adding yourself to the book, even if you plan on taking 80 years to do it (hey, that's only 28 miles a year!).
The bookstore/gift shop is top notch - maps and guidebooks for every section of the AT to a plethora of AT Thru-Hiker Memoirs to t-shirts!
The folks that work there are always friendly and helpful. I've seen them take the time out to talk to Boy Scouts (thanks for letting us invade your store), to answer questions about where to go hiking nearby or far away, help iron out a hike plan, or to simply find out where you've been so far and what it was like. It's been a great experience every time I've gone.