What it is: Joomla! is an open source content management system offering many extensions to add features to the site.
Type: Open source
Cost: None
Features: Joomla! offers marketing and e-commerce tools, along with extensibility and a large support community.
Pros: Has a great community, strong framework and extensibility.
Cons: The built-in, core functionality sometimes can be flawed, or limiting.
More about Joomla! here.