Google has announced Page Speed Tool at 2011 Google IO conference and then refined Google's index algorithm Caffeine in June 2011. A lot of factors has been changed since then, but a very important and obvious factor affect search rank is now page loading speed. You can use Google page speed to measure how fast your web pages are. also it gives some useful suggestions for you to optimize your web pages.
Another good stuff from Page Speed project is an module for Apache Web Server. Google post a video to show the difference between before and after using this module.
http://code.google.com/speed/page-speed/docs/using_mod.html
Google page speed
websitegrader.com
websitegrader is another tool i found useful. It's similar to Google Page Speed. Basically, it scan your website first and gives you a score to present how well your website is doing and also gives you optimization suggestions.It focus on social media and pages links in a lot, which is another major factor affecting search engine ranking.
http://analyze.websiteoptimization.com/wso
websiteoptimization.com provides another tool to provide code optimization on your website. It's very similar to Google Page Speed. It mainly focus on web page code quality.
Other good references:
Google SEO Guide
