jsFiddle - Web Design Tool

[Taxious]

This looks pretty sweet - it gives you a separate input field for html, css, and javascript then shows you the result in another window. It even lets you choose from a couple different javascript frameworks on the fly.
http://jsfiddle.net/

For example, here is one that plays around with jquery and some images:
http://jsfiddle.net/marcofucci/rRtAq/