Free tools
Free SEO tools that answer one question each
Each of these takes one address and answers one question, properly, with the reasoning shown. No account, no trial, no credit card, and nothing held back for people who sign up.
They read the page as it is served, the way a crawler sees it, without running JavaScript.
robots.txt tester
Read a site’s robots.txt, say whether a URL is allowed for Googlebot, and flag the directives that do nothing.
XML sitemap validator
Find a site’s sitemap, check it parses, and report the entries search engines will reject.
Redirect chain checker
Follow a URL hop by hop and show every redirect between the address you typed and the page that answers.
Hreflang checker
Read the hreflang annotations on a page and check the language codes, the self-reference and x-default.
Title and meta description length checker
Measure a page’s title and meta description in characters and in pixels, and say what a result will show.
Canonical tag checker
Find the canonical a page declares, in the HTML and the headers, and check it against the URL that served it.
Image alt text auditor
List every image on a page with its alt text, and pick out the ones that are missing, empty or useless.
When one page is not the question
These tools check a single address, which is the right size for “why is this page not indexed” and the wrong size for “what is wrong with this site”.
For the second one, the Site Spider crawls up to 5,000 pages and runs fifteen checks across all of them, then hands you the whole crawl as a SQLite database so you can ask questions nobody built a tool for. Still free, still no account.
If you are starting from the beginning, the DIY SEO guide puts these checks in the order that pays off.