SEO is important, and keeping tabs on how your website or app is doing is important too. So (in no particular order) we've put together a short list of websites that you can use to get a look at your sites strengths and pitfalls.
You'll find that most SEO checkers now allow you to enter multiple websites and compare their strengths and weaknesses in SEO, but this one does it nicely. SEO Site Checkup lets you compare as well as view results individually.
This one might not look as nice as the others, but the advice it gives is second to none. After checking your websites SEO characteristics, it will then go on to suggest how you can improve in those areas, or fix any problems it might find (as well as let you know how important any pitfalls it finds may be).
Simple and to the point, HubSpot's Website Grader gets the job done quickly and looks good whilst doing it. After giving your wesites URL and an email address, your site is checked for everything from load speed, to how secure it is to your customers.