Chamilo is one of the top 3 Open Source Learning Management Systems, and the easiest to use, from what our most experienced users tell us.
It is available in 40+ languages.
Anyone can easily install Chamilo on a web server, create courses, upload content, subscribe users and get teaching.
It offers dozens of options to create documents, tests, forums, learning paths, wikis, etc, from super basic to super advanced mode, but also to import content in SCORM, QTI, Aiken, AICC, Common Cartdrige or share activities in xAPI & LTI.
Chamilo has demonstrated capabilities of up to 1M registered users on a single server instance, and we've got it up to 5,000 simultaneous users taking exams at the same time.
Reliable, highly configurable and very lightweight.