1) Google Page Speed
"With PageSpeed Insights you can identify ways to make your site faster and more mobile-friendly."
https://developers.google.com/speed/pagespeed/
https://developers.google.com/speed/pagespeed/insights/
Note : Perfect to find problems related to fonts, image weight, etc...
2) Mxtoolbox
"All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool. Input a domain name or IP Address or Host Name. Links in the results will guide you to other relevant tools and information. And you'll have a chronological history of your results."
Note : Perfect to find NS problems, bugs, etc...
https://mxtoolbox.com/
https://mxtoolbox.com/NetworkTools.aspx
3) Pingdom
"Enter a URL to test the load time of that page, analyze it and find bottlenecks."
https://tools.pingdom.com/
Note : Perfect to find problems on your website and boost your website.
4) Nu Validator
"Validate your source code."
https://validator.nu/
https://checker.html5.org/
Note : My preference goes to checker.html5.org , because it indents visually h1 > h6
5) Cloudflare
"Make the Internet work the way it should"
https://www.cloudflare.com/
Note : Boost your website with caching and add https to your website.