Official Grand Canyon site. Online permit forms, news, data, tips, etc.
JMT trip report and photos from fellow JMTers David and Rebekah, including detailed trail profile, gear list, etc. Extremely thorough. Check it out!
The John Muir Trail. Pages dedicated to everything you need to know to plan your own JMT through-hike. Or at least dream and drool over the possibilities. :-) The transportation discussion is outdated however. See below - life just got much easier for us!
YARTS: Regional Yosemite shuttle service. Finally! Use it, love it. ;-)
The JMT Journals. Journals and progress updates of JMT (John Muir Trail) through-hikes.
Pacific Crest Trail Association. Dedicated to the PCT, the one mega-trail I will through-hike before I croak. Hopefully the others too, but ya gotta start somewhere. ;-)
Continental Divide Trail Society. Help maintain the CDT - nearly 3000 miles of trekking through the Rocky Mountain states, from Mexico to Canada.
Appalachian Trail Conference. Maintainers of the 2,167 miles of the AT.
Appalachian Trail Home Page. Very nice site by Kathy.
North Country Trail Association. 1,500 miles completed and another 2,500 to go before the North Country National Scenic Trail is completed, hugging the US/Canadian border states from North Dakota to New York.
FallingWater.com. The Moak family's tales and photos from along various Big Trails.