Meta Tags - Meta Tags is a tool to debug and generate meta tag code for any website.
DevDojo - Dev tools and resources for building awesome stuff
Devhints - TL;DR for developer documentation
UXsniff - Track user behavior on your website and improve conversions with UXsniff: the user experience watchdog that looks out for you.
OpenGraph.xyz - Check how search engines and social medias such as Google, Facebook, Twitter display your website
UX Recipe - Checklist to discover your next UX project tools/techniques