Trailplace
Dan "Wingfoot" Bruce is, do not doubt it, the #1 expert on the Appalchian Trail today. Having hiked it 7 times, and as the author of the Thru-Hiker's Handbook, he knows his stuff. His website, Appalachian Trailplace, is probably the single best source of information on web about the Appalchian Trail. His site is definitely targeted at thru-hikers, though it is quite helpful to people hiking shorter distances, too. People planning a thru-hike should absolutely sign up for the Appalchain Trail Mailing List. I subscribe.

Applachian Trail Homepage
Kathy Bilton has maintained this website before there was a single other AT-related site on the web. It's not the most glamorous, nor is it the most interactive, but it's updated quite frequently, and always gets you the info that you need.

The Appalachian Trail Conference
The ATC is the group that maintains the Appalchian Trail, takes care of the legal and political side of things, and is, for all intents and purposes, the governing body of the AT. Located in Harper's Ferry, West Virginia, their headquarters are known as the "psychological half-way point of the AT".

The Utimate Trail Store
They're getting a little ahead of themselves with the name, but they do sell a good amount of AT-related stuff. This site is most useful for ordering maps and guidebooks. But skip their thru-hikers' guide and go with Wingfoot's.

The Kushmans Hike the AT
The Kushmans hiked the AT from Georgia to Harper's Ferry in 1996, carrying a Zaurus palmtop and updating a journal. As they started hiking about a month before me, they were technically the first folks to hike with a computer. But some would argue that their system wasn't really a computer. Personally, I'm not sweating it. :)

Linda and Ron Moak's 1977 Southbound Journals
This is an very well-done web site that documents the Moaks' trip from Maine to Georgia. Each day there is a journal entry from each, giving two different perspectives on the day.