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.
Amino Editor - Amino is a Chrome browser extension for customizing web page presentation with user CSS.
CodePen - A front end web development playground.
Stylus - User Styles Manager - Stylus is a userstyles editor and manager based on the source code of Stylish version 1.5.2.