HubSpot provides this useful tool free (presumably to sell you on their paid service) which analyzes your website and grades it based on SEO criteria like Keyword quality, alt tags, directory listings, etc. Most criteria are well accepted, so the report is a useful checklist. However, if you're relatively sophisticated on SEO tactics, websitegrader is unlikely to tell you anything you didn't already know about your own site. Websitegrader is perhaps most useful at competitive intelligence gathering - that is, comparing your site's SEO attributes with those of competitive websites.