Meta Tags - Meta Tags is a tool to debug and generate meta tag code for any website.
Topstyle - TopStyle contains powerful tools for building standards-compliant web sites.
Devhints - TL;DR for developer documentation
Rapid CSS Editor - Rapid CSS editor is quick, powerful and easy to use HTML and CSS code editor for web developers. Intuitive interface and intelligent features help you become more productive and create modern HTML5 and CSS3 based websites faster and easier.
OpenGraph.xyz - Check how search engines and social medias such as Google, Facebook, Twitter display your website
EnjoyCSS - CSS generator for rich graphical styles without coding