I've only recently become aware of Google's Page Speed Firefox add-on integrated with the Firebug extension.
Page Speed is Google's equivalent of Yahoo's YSlow, also a Firefox add-on that needs the Firebug extension to work.
Like YSlow it tests the performance of your webpages against a set of criteria, giving an overall score for a page as well as advice on how you can improve your page performance.
Not surprisingly many of the recommendations are similar to Yahoo's, but some are different, giving you an additional perspective on website performance.
referenced from a page when you run the analysis, saving them to a local directory. Drilling into a rule, such as Minify CSS tells you the potential reduction in size as well as a link to the optimized version.
Page Speed also has mod_pagespeed, an open-source Apache module for Apache 2.2 that automatically optimizes
web pages and resources on them. I haven't investigated this, but it would be interesting to experiment with.
I use both YSlow and Page Speed, each tool has something unique to offer. Page Speed is a useful addition to your toolbox for web development.