Speed testing your web site with AOL Pagetest
This site accepts a URL which it loads over a network connection with the speed of your choice. After loading the site it offers you a waterfall graph of page elements, a screenshot so you can see if it loaded correctly, and automatic comparisons to previous page loads to see how caching affects your site.
The code behind the page is based on the open source AOL Pagetest which means it can be modified as needed. I think I heard Ryan say something about automated testing over time with historical graphs and email alerts? :)