Meta Tags - Meta Tags is a tool to debug and generate meta tag code for any website.
Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
Devhints - TL;DR for developer documentation
FreeStylerWs - Styles and skins for all popular sites in the web
Meta Tag Generator - Generate HTML code optimal for SEO, social media, & mobile.
Stylem - A user style manager for Pale Moon, Basilisk, Interlink, SeaMonkey, and other Mozilla-based software. Install styles from userstyles.org to change how web pages and the application look.