ReText is a simple but powerful text editor for Markdown and reStructuredText. It has live preview allowing you to see how your text will look like right as you type. You can export files to Google Docs, Dropbox and Box.
In case you're not familiar with Markdown, it is a markup language that allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML or HTML.
Browser requirements: Chrome, Firefox, Safari.
Nothing exciting, but works fine