I am not much of a web guy at all, but lately I had no choice. Had to work on a small project and I was forced to look into a good CMS. I went through a lot of them. Mostly php based ones, which are very famous and strongly suggested by my fellow friends. They were all cool.
But then I got to plone. Plone is a open source cms built on top of zope. I think this is the best CMS I have seen thus far. It is more then just a simple CMS, there is a hell lot more to it then being a simple CMS. I also liked the fact that I can develop stuff for it, more like addons, using python and take advantage of zope application framework. I had to less concentrate on the presentation layer and could do more on the business logic of the stuff I worked on. I really enjoyed the last 3 months I spend working with zope and plone.
Also it is worth to note that companies like Lufthansa, Nokia, eBay and also Nasa have adopted plone on various levels. I think even Ubuntu is using plone on their web site.