Accessibility
Developer Resources (from Webnet)
Watchfire WebXACT
Web Accessibility at Berkeley (pdf)

Current Awareness
CARL-IT North (blog)
Library Weblogs (list of blogs and RSS feeds)
Web4Lib Electronic Discussion

HTML
EchoEcho.com
HTML validator
W3schools

Usability
Designing Interfaces
Usability in Website and Software Design
useit.com (Jakob Nielsen's website)

Web Design
Developer Resources (Webnet)
W3c

Writing for the Web
How users read, and how authors should write

XHTML
W3schools
WebHeadStart.org
XHTML Guidelines & Benefits (NYPL)