I was tired of firing up Firefox just to freely edit CSS and have it update the page in real time, so I came up with this extension for both Chrome and Safari
Safari: Download for Safari
Chrome: Download for Chrome
Once you have the extension installed, use the toolbar button to open up the editor. You can resize the box by dragging the textarea handle.
The code for the extension is available on Github: Safari Live CSS Extension.
If you have questions, comments or suggestions I would love to hear them.
@jhubert on Twitter.