Feature-Rich, Robust & Professional CMS: Vivvo
Vivvo is a professional and "commercial open source" CMS application that is built with PHP, uses MySQL for storing data and offers a "top-notch" experience.
The application is ready to power any type and size of website from a portal to a newspaper, a blog, etc. as it is already preferred by 1000s of websites.
The CMS comes as a downloadable package to install on your own servers (has a web-based installer) with powerful features for managing the content like publishing articles, static pages, blogs, images, photo galleries, videos, podcasts and keeping revisions of them.
It has the tools to equip any website with functional modules like an event calendar, commenting system, integrated search, ability to show related articles and integrate with popular forum softwares.
Also, the application helps the content to spread with social bookmarking syndication, "email to a friend", print version, subscribe, RSS and more.
Vivvo has an impressive and slick admin interface which has a permission-based user management system where it is possible to define editors, writers, photo gallery managers, members, etc.
It is SEO-friendly in means of the URLs created, ability to customize meta tags/keywords and auto-sitemap generation. And, the 20 free templates that the CMS comes with are all SEO optimized.
Besides these features, it is always to think of something missing. But Vivvo is ready to cover it with 100+ free plugins and widgets (like multi-site manager, Twitter updater) where it is possible to create new ones as the application is open source.
Vivvo is very well documented with user guides, a knowledge base and a highly active community behind it.