Plone is a content management system built on CMF (Content Management Framework) which in turn is built on the Zope web application framework. It is python based and is quite different to the other CMSs that I have installed that are based on php and perl. Zope has its own webserver and object orientated database system unlike the others which use apache and mysql or postgres. Zope will however work with Apache or any other web server that includes Common Gateway Interfaces (CGI). It will also interface with other relational databases if you would rather store all or just some of your data in those. It is supported by Zope Corporation and has a large team of developers from around the world contributing. There are a large number of modules available and I read accounts of it handling 900 hits per second.
Plone is relatively new but has had some very good reviews and is built on zope a highly regarded framework.