I was putting together a list of resources that I regularly use for someone, and I thought it might be helpful for someone else.

Firefox Resources

• Web Developer -  all purpose testing. Extremely helpful when checking CSS, but also good with images, forms, and other HTML elements.
• Firefox Accessibility Extension - evaluate accessibility on web sites and other HTML documents
• WAVE Toolbar - another good accessibility testing tool
• IE Tabs Plus - open IE within a Firefox tab
• LinkChecker - the name says it all
• MeasureIt - find dimensions of online content or space
• ColorZilla - find the exact color you're seeing in the browser.
• HTML Validator - a great validation tool for testing web pages.
• Fangs Screen Reader Emulator - emulates a screen reader for testing accessibility and other usability factors.
• Split Browser - open two pages in the same window.
• Live HTTP Headers - know what your page is loading, and from where
• FireShot - superb screenshot tool.
• Firebug
• Colour Contrast Analyser
• Multi Links
• Other helpful, extensions
  • All-in-One Gestures - makes browsing easier.
  • Profile Manager and Synchronizer - open multiple Firefox profiles at the same time.
  • Scribefire - designed as a blogging tool, it's also helpful for just collecting links and content in clean HTML.
  • IE View - switch from Firefox to IE in a click. No need to find the page again in IE, just open your current Firefox page in IE.
  • FoxView -open your current IE page in Firefox
  • OperaView - open your current Firefox page in Opera
  • Chrome View - open your current Firefox page in Chrome
  • Safari View - open your current Firefox page in Safari
  • Pencil Wireframing Tool

Resources for Internet Explorer

• Internet Explorer Developer Toolbar - Now built into base browser.
• Web Accessibility Toolbar for IE

Resources for Opera

• Opera Browser - A resource all by itself. If you're not familiar with it, it's another high quality browser that's useful to test pages in.
• Web Accessibility Toolbar for Opera

All Browsers

• xmarks - extensions available for most browsers.
• extension for government video