Update: Speed up Apache - how I went from F to A in YSlow. Good example of using YSlow to speed up a website with solid code examples.
Every layer in the multi-layer cake that is your website contributes to how long a page takes to display. YSlow, from Yahoo, is a cool tool for discovering how the ingredients of your site's top layer contribute to performance.
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: