The Customer Focus Calculator is a great way to perform a quick check of your content, and whether you’re speaking to yourself instead of your customers.
Hemingway (yes, that Hemingway) is a simple, clean way to check the grade level and effectiveness of your writing. They also have a handy Mac app that I use all the time.
Moz – This is the place to get started with SEO. If you don’t know anything, or if you think you know everything, you’ll find plenty to learn.