Core Must-Haves

• http://validator.w3.org/ : Validate your HTML early and often.
• Evolt Browser Archive : Pretty much any and every browser plus version history ever created can be found here.

Accessibility

• Validate for Accessibility (partial, automated) : WAVE
• Validate for Accessibility (partial, automated) : WebXACT
• W3C Guidelines (practically impossible to meet, but a worthy goal) : http://www.w3.org/TR/WAI-WEBCONTENT/

Good Resources

• http://developer.yahoo.com/yui/ Cross-browser compliance. Great drop-in interface tools. Toys. The Yahoo User Interface Library rocks. It also means you can get home faster at the end of the day. Stuff like: http://developer.yahoo.com/yui/examples/container/panel_clean.html and http://developer.yahoo.com/yui/grids/builder/
• http://css-discuss.incutio.com/

Firefox Authoring Tools

• Aardvark Firefox Extension

Bookmarklets
(add these to your Links / Bookmark Toolbar)

• Resize Browser Window to 640 x 480 (great for rough and quick testing for various screen resolutions)
• Resize Browser Window to 800×600
• Resize Browser Window to 1024×768
• Ruler (IE Version)
• Don’t add this link here, go to the website: Ruler 1.2 (Firefox Only)
• Validate Markup
• Validate CSS
• Check Links
• Toggle CSS Style Sheets (good for viewing logical structure)