Some background on this: our prototyping tool allows you to write code in the browser, and the way maney people use it is they open a prototype on one monitor and then have the editor on the other one. So you save, alt-tab, refresh, just like you would if you were writing code offline.

After seeing LiveCSS (https://github.com/ooyala/livecss) we figured we could take the idea a step further and get rid of the need to alt-tab and refresh in the first place. It's a simple gimmick, but in combination with our prototyping app, it suddenly (in our eyes) becomes a lot more interesting. Why? Because your client can watch changes happen as you make them. You don't have to sit there on the phone/IM saying "refresh it now. Ok, how about now?" - they just see things change. Especially if they use a browser that supports transitions - it's way cool to see your design morph back and forth as you consider various options.