Plone is a fantastic CMS (Content management System) running on Linux and OS X. Plone gives clients a professional framework on which to work and customise. It offers one of the best ways I've found to give users the ability to customise their own content without the need for complex custom applications or large amounts of training.

Of all the CMS OpenSource systems out there Plone is one of the easiest to get to grips with and install onto standard hardware.

Great for both internet and intranet use and because of it's simple installer it can be up and running in a very short time and without having to provide a dedicated server.