I've recently been involved in more and more SEO work and am find it pretty rewarding if you can improve a site's position in google.
My site improves daily as I add and improve the page code and content. Anyway, in the course of my research to find useful tools online, I came across an extremely useful online set of tools to test the SEOness of a site.
It checks on speed, optimisation of images, CSS and loads of other stuff. So, if you are wanting to SEO your site this is definitely worth checking out... www.feedthebot.com