YSlow - Come together with FireBug for FireFox extension and developed by Yahoo!. It provides you with Web traffic analysis as well as performance tips. Abstract from YSlow
YSlow analyzes web pages and tells you why they're slow based on the rules for high performance web sites. YSlow is a Firefox add-on integrated with the popular Firebug web development tool.
YSlow gives you:
- Performance report card
- HTTP/HTML summary
- List of components in the page
- Tools including JSLint
Fiddler - Web trace tools develop for IE. It provides web debugging and analysis. The latest version claims to have FireFox Hook. Abstract from Fiddler
Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.