Onpage.org is a tool developed in Germany, and extremely relevant for on-page SEO, and I highly recommend this service, especially since you can use it for free! The developers of Yoast SEO (the nr.1 Wordpress SEO extension) even decided to integrate the service in their plugin, which proves how relevant the service is. Onpage.org offers a free account for 1 small website (max 100 URL's). Just register your site and email address, and you get a very thorough audit. A few hours after signing up you get the credentials to sign in to the dashboard and take a look at your audit. The first time I checked it I was quite stunned. This is the most extensive on-page SEO audit that i've come across, and I've seen a few.... Many other services offer on-page audits as part of their packages, but this one beats them all. 

The Onpage.org dashboard

The dashboard gives you a nice overview. What immediately strikes is that the tool really discovers many more URL's then you knew you had. It also includes any redirected URL's you have, so you can always check any URL and to where it redirects, etc. Then you get a very long checklist of many on-page paramaters, and how well they are optimized. I tried to crank the dashboard in a not too massive screenprint here:

Also a list view with the most critical issues is provided:

You see, a lot to check. There is a lot of information about the status of your website that other tools also provide, but then Onpage.org just seems to go a lot deeper. As an example, a iusual audit for metadescriptions checks the character count. This tool does that too, but they provide a pixel count as well, which is sometimes usefull for mobile searches. All options are available from the checklist, but the sidebar navigation also provides an easy navigation-panel, including dropdown options for even more details:

 

 It goes to far to cover everything, so let me just pick a few random items to demonstarte the tool.

Page inpector

From any overview, you can always click the Page inspector, which brings up a detail overview for this specific page, with score per individual paramater:

Duplicate content audit

A unique feature is the duplicate content audit. It performs both an exact duplicate content audit as well as a near-duplicates audit. This gives you a pretty good idea whether any page has pages resembling it closely:

With my audit it indeed found 3 duplicates! I already knew these, and configured canonical URL's, but if you are not aware yet, this is perfect.

Canonical URL audit

Speaking of canonical URL's, of course there is an option for that as well. It turned out pretty useful for my site, as I recently switched my site to SSL, and I had canonicals URLs pointing to non-SSL URL's in a few cases. Easily discovered with this tool:

Clicking on any of the graphs brings you to the corresponding overview, like an oberview for pages without a canonical, or incorrect ones.

Sitemap check

Onpage.org also checks your sitemap. Of all URL's discovered, it also indicates whether they are included in the sitemap or not. Often the reason for them not to be included is valid, but with this overview, you can very quickly check if this is indeed the case.

HTTP status codes

Easily check your HTTP status codes. Especially tracking codes like 404 or 303's is very easy now:

Conclusion

Overall, this is really a stunning overview and it's completely free for small sites. The Pro-version of the tool is not cheap, starting at €99 euro or $119, but you've got a great tool then with capacities for multiple websites. You can sign up from Onpage.org