E.g. If I wanted to show someone an idea I had for tweaking the NYTimes.com layout, I would go to this web app, put in the the URL http://www.nytimes.com, have a textarea where I could override the CSS, and then send a preview link to someone that loads the NY Times home page with my CSS modifications?
Have a look at CSSEdit. I've never actually used it, but I think it fits your description.<p><a href="http://macrabbit.com/cssedit/" rel="nofollow">http://macrabbit.com/cssedit/</a><p>Edit: Not sure about sharing results though