Gregory (packs)
Mountain Smith (packs and bags mostly)
REI (EVERYthing)
Sierra Designs (tents, clothing
The North Face (tents, bags, clothing, packs)
Princeton Tec (lights)
Smartwools (socks)
Pur (Water Filters)
Aqua Mira (Water Purification Solution)
Cascade Designs (thermarest, pack towels, platypus)
Camelback (bladders, hydration packs)
Nalgene (containers)
Ursack (bear proof containers)  Heard a lot of mixed reviews.  Do the newer models work?
Frogg Toggs (raingear)

Hints, Tips, Reviews, Hiker and PCT specific Websites

Joe's Ultralight Backpacking (usefull info for the ultralight crowd)
Chris Ibbeson's Gear Weight Calculator
Craig's PCT Planner (PC application for planning resupply stops)
Pacific Crest Trail Association (permits, membership, books...)
Live to Play gear reviews (TONS of reviews)
Outside Online "Gear Guy" ('tips and tricks')
The Pacific Crest Trail Hiker's Website (hiker registry, forums, etc.)
Desert Portion Website
"What is a Thru Hiker" by Jim Owen  (i really like the way he puts this...)
Hikertrash (maps)  I can't get the maps to work.  Myabe you'll have better luck.
National Scenic Trails Mailing Lists (PCT, AT, CDT, and more)
Peak to Peak (a listing of various hiker's pages)
Sierra Wilderness (info you ought to know...)
US Army Research Paper on Nutrional Guidance in Temperate and Extreme Conditions (PDF file.  EXCELLENT Info!)

Multiple Brand/Product Retailers (Online)

Backcountry Gear Limited (this one seems to have just about everything I want at really nice prices)
The Tent Store (take a guess)
Trailquest (lots of those little, harder to find items...all conveniently sold here)
REI (EVERYthing at decent prices usually.  better deals can usually be found however)
Campmor

Books

Pacific Crest Trail Data
The Pacific Crest Trail: A hiker's companion
The Pacific Crest Trail: Southern California 
The Pacific Crest Trail: Northern California 
The Pacific Crest Trail: Oregon and Washington 
 Journey on the Crest : Walking 2600 Miles from Mexico to Canada (More of a book about the author and people than the trail itself)