In a word, Joomla! is FANTASTIC. It's used to power tens of thousands of websites, from personal blogs to huge corporations. A website powered by Joomla! is self-editable, meaning that you - rather than a web designer or developer - can manage the pages (content) yourself. And because it's open source software, Joomla is free to use and is continuously developed and upgraded by hundreds of different people.
Zen Cart is a free, database driven, open source shopping cart system that is probably one of the most flexible, user friendly yet powerful shopping cart programs available. Zen Cart™ is built for entrepreneurs by entrepreneurs. The program is built by programmers and designers who understand the online selling process from start to finish. Zen Cart™ has included all the necessary tools required to build a successful online store, and developers are constantly improving and upgrading the software based on users' requirements.