YSlow

YSlow is a Firefox extension created by Yahoo! developers that integrates with Firebug (therefore you need to have Firebug enabled for it to work). YSlow analyzes a Web page for front-end performance and, in its simplest usage, gives you a letter grade (A being the best and F being the poorest) for each of the best practices for speeding up  your website.

YSlow also allows you to inspect in detail things that are essential for a high-performance website. For example, the Stats view gives you the total size of a Web page and a summary of items that are loaded when the Web page is requested (i.e. style sheets, JavaScript files, Flash objects and images), so that you can hunt down the bottlenecks that cause a Web page to load slowly.

The Components view outlines every single component of a Web page in tabular format and allows you to inspect it to see attributes such as size, expiration date (for cached files), whether it uses server-side compression (Gzip) and response time (how long the component took to load).