CODECAST - HTML & CSS editor in your browser with real-time previews
Stylebot - Change the appearance of websites instantly. Preview and install styles created by other users on stylebot.me
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
Dark Reader - Reduce eye strain in your browser with this extension that provides a dark theme for browsing.
CodePen - A front end web development playground.
FreeStylerWs - Styles and skins for all popular sites in the web