An excellent resource for testing and optimizing Web site performance. The information contained in the results is the most thorough of any I've seen.
Dave Rupert provides an excellent breakdown of how to speed up your WordPress-powered site. There's no excuse to not spend the time implementing these changes. Even if you're like me, and have already done a lot of the work, I'm willing to bet there's a step or two that will make your site even better. For me, it's the super-duper-easy cut-and-paste improvements to the htacess file to enable gzip and browser caching.