These are the sites I come back to repeatedly for help with layout, code snippets, reference, and more.

User experience design, general

• Boxes and Arrows
• UX Matters
• SlideShare (for presentations about design, IA, etc.)

Interaction Design

• IXDA discussions
• Designing Interfaces
• Designing Interactions

Visio Stencils and Methods

• Information Architecture Institute: stencils, presentations, templates
• GUUUI.com: Visio – the interaction designer’s nail gun
• Jesse James Garrett: A visual vocabulary for describing information architecture and interaction design

CSS Layouts

• Accessify Tools & Wizards
• Max design
• Free CSS Templates
• Inknoise layout-o-matic
• Code sucks layouts
• Dynamic Drive CSS

Javascript & AJAX

• Dynamic Drive scripts, image optimizer, favicon generator
  
• Javascript forms FAQ
• Scriptaculous demos
• Mootools
• Dojo

Dreamweaver Resources

• Adobe Dreamweaver Exchange

Photography

• PDPPhoto
• iStockPhoto

Fonts

• Urban Fonts (free)
• What the Font
• Identifont
• Dafont.com

Icons and Backgrounds

• Free Silk icons (FamFamFam)
  
• Pixel patterns
• 50 free icon sets

Validation and Standards

• W3C XHTML validator
• W3C CSS validator
• Cynthia Says accessibility checker
• WebXACT accessibility checker

Other useful sites

• Evolt browser archive
• Em and En — correct web typography
• Lorem Ipsum generator
• 100 PowerPoint tutorials