Google just has open sourced Page Speed, a tool used internally by Google to optimize their web sites, especially the speed web pages are loaded with. Page Speed is an open-source Firefox/Firebug Add-on. Webmasters and web developers can use Page Speed to evaluate the performance of their web pages and to get suggestions on how to improve them.

By using Page Speed, you can:

Make your site faster.Keep Internet users engaged with your site.Reduce your bandwidth and hosting costs.Improve the web!Through Page Speed, during development of web pages, Google suggests:

• Avoid CSS expressions
• Combine external CSS
• Combine external JavaScript
• Defer loading of JavaScript
• Enable gzip compression
• Leverage browser caching
• Leverage proxy caching
• Minify JavaScript
• Minimize cookie size
• Minimize DNS lookups
• Minimize redirects
• Optimize images
• Optimize the order of styles and scripts
• Parallelize downloads across hostnames
• Put CSS in the document head
• Remove unused CSS
• Serve resources from a consistent URL
• Serve static content from a cookieless domain
• Specify image dimensions
• Use efficient CSS selectors

As commented by some testers, it detects more defects than YSlow. Hope you find it helpful.

Please find source at attachment.