Overview of Plone and its features. It is the online Plone brochure.
Community-maintained manual for the Plone content
The central place for discussions, questions and community communication.
The curriculum used for Plone trainings, also useful as a learning resource on its own.
The Plone community adheres to extensive testing while developing software, and uses Continuous Integration. Here you can find the latest test runs, managed by Jenkins.
Plone is fully open source, and is free software under the GPLv2. On Github you can find the many repositories that make up the Plone project, ranging from the core software itself to the documentation sources.