markItUp! is a JavaScript plugin built on jQuery which can turn any textarea into fake "rich" markup editor for Textile, Markdown, BBcode, MediaWiki, Html, etc -- with built in AJAX preview...
Pastebin++ for web technologies: paste javascript, css and html and get a rendering of them all at once. Create packages and version them, publish to your blog, etc. MooTools, jQuery, Prototype, YUI, Glow and Dojo all supported.