Plone is an add-on to Zope that is growing very popular in the Zope community developers. Actually, on could say that Zope is the platform and Plone is its killer app.
I'm personally not happy with the default design of a Plone site. I think it's not user-friendly. However, it's relatively easy to change because it's skinnable.
Here's one such site that I must admit looks great: Andy McKay's website for his company ClearWind. Lovely!
