Here are a few useful tools that our clients like to use when testing their own site or a few tricks that your developers might like. These help take the mystery away from what's behind that pretty website:

Validate Code

• W3C HTML Validator

Website Speed Optimization

• Yahoo's Yslow is a browser plugin that lets you analyze why any public website is loading slowly. 
• Website Analyzer is a free online tool that analyzes the download speed of any website.
• Optimize your CSS by minifying it with this online CSS Compressor.
• Don't forget to minify your JavaScript also.

Accessibility

• 508-compliance testing at http://www.cynthiasays.com
• Section 508 Technology Tools
• W3.org 508 Cheat Sheet
• Website Accessibility Testing Tool by PowerMapper

HTML5 Videos

• Player with code detection for WEBM, OGG, and M4V formats, including Flash degredation option for older browsers: VideoJS
• Open sources video encoder: Miro Video Converter

Cloud-based Editing Tools

• Icon creator that supports transparency and file uploads. Includes a nice preview interface: X-Icon