Adobe Studio (collection)
Apple Masters (collection)
Russell Brown
John Paul Caponigro
Digital Renaissance (collection)
Katrin Eismann
Diane Fenster
Auriea Harvey
Ryszard Horowitz
Catherine McIntyre
Diana Meredith
Bert Monroy
Jeff Schewe
Maggie Taylor
Laurie Thompson
Jerry Uelsmann
Greg Vander Houwen
Ben Willmore

Adobe
the mother ship

BoxTop Software
PhotoGIF, ProJPEG, ImageVice and more plugins

Extensis Corporation
lots of imaging software, including PhotoFrames

Xaos Tools
Paint Alchemy, TypeCaster, etc.

AutoFX
edge effects, other imaging software

HumanSoftware
Squizz, Textissimo, PhotoSpray and more

The PhotoshopCafe
tutorials, tips, actions, etc.

The Pixel Foundry
home to Kai's Power Tips and Tricks, and much more

Team Photoshop
Ask Mr. Photoshop your questions!

Planet Photoshop
news, tutorials, editorials, seminars, and more

Binary FX - Photoshop Tech Vault 2.0
intermediate to advanced, leans more towards prepress issues. FABULOUS color correction info!

Hands On Training by DocOzone
walk-throughs of common effects

Training and tutorials sponsored by Sterling Ledet & Assoc.

Adobe Evangelists Tips, Trick, Tech

Retouch Pro for those interested in photo retouching and restoriation

Jay Arriach's Photoshop Tips for beginners

About.com has several references:
Graphic Design
Web Design
Graphics Software

Photoshop Web Reference
A source of a relatively complete web reference for all things Photoshop (not updated much since v4)

Production Graphics at Webreference
Wendy Peck's great tips, tutorials and columns

PC Resources for Photoshop
now offering Mac filters too

Adobe Photoshop Listserv Information
one of the quality PS lists

Out-Opts
for all your output questions and resources

National Association of Photoshop Professionals
great benefits to members, including excellent videos and publications

PEI Magazine
great tips, ideas, articles each month

Computer Arts
what the Brits are up to. GREAT articles, tips, tutorials, galleries - more than just Photoshop.

HOW Magazine
"The business, creativity and technology magazine for graphic designers"

In no particular order, depends on what you need to do....some have been updated to the CS version:

Hands On Photoshop 7 Tutorial Workshops - Stan Shire
Adobe Photoshop 6.0 - Classroom in a Book - Adobe Press
Photoshop 6 Wow! - Jack Davis, Linnea Dayton
Photoshop 6 Down and Dirty Tricks - Scott Kelby
Photoshop 6 Artistry: Mastering the Digital Image - Barry Haynes, Wendy Crumpler, Sean Duggan
Photoshop 6 Studio Techniques - Ben Willmore
Photoshop Restoration & Retouching - Katrin Eisman
Real World Photoshop 6 - David Blatner, Bruce Fraser (the Pixelboyz)
Adobe Photoshop 6.0 for Photographers - Martin Evening
Photoshop Studio Secrets and Photoshop 6 Bible - Deke McClelland
Professional Photoshop 6: The Classic Guide to Color Correction - Dan Margulis
New Masters of Photoshop - Mike Cina, Mike Young, Eun-Ha Paek, Jens Karlsson
Adobe Photoshop Master Class - John Paul Caponigro

more can be found at the Designer's Bookshelf

The Action Exchange
from Adobe, actions, extensions, and more for your Adobe software

ActionFX
Al Ward's Actions Resource Center

DigitalThread: Photoshop Action Script Archive
your "designer drug store" and more