Just as I was looking at the Webmasters tool today I found a new FireFox add-on tool by Google called Page Speed. It helps you evaluate the performance of your web page and get suggestions to improve the speed/performance of your web page.
The below video gives you a quick overview of the tool which showcases some cool features, like javascript minify and others.
Another alternative to this is YSlow another FireFox add-on from Yahoo, to increase the performance of your webpage