Meta Tags - Meta Tags is a tool to debug and generate meta tag code for any website.
Nibbler - Free tool for testing how good your website is, and what you can do to improve it. Check accessibility, SEO, social media, compliance and more.
Devhints - TL;DR for developer documentation
Colorific - Colorific Hair Color Salons give you amazing color at an affordable price.
Meta Tag Generator - Generate HTML code optimal for SEO, social media, & mobile.
HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.