I love taking tests. Sounds weird, I know, but I love learning, and tests are a quick way for me to discover what I don't know and immediately pinpoints an area or two that I can improve on. And I found a website SEO tool that tests your website and gives you a grade. Sandstorm Design got 99/100!
When it comes to SEO and marketing your website, the high level concepts don't often change (write strong original content, continue to add to that content, attract quality links, etc). What does change are all the new things you can do to increase your odds that people will come back to your website and promote your content, which in turn drives more traffic. Of all the website SEO tools, I like Website Grader. (and no, they didn't pay for that link).
When I find a tool I like, I share it. And what I like most is that Website Grader put together in a very organized fashion what they look for in the analysis, what you can do to improve your website, and if you want more help, where to go. And it's free.