Moodle

http://moodle.org/

"Moodle is a course management system (CMS) - a free, Open Source software package designed using sound pedagogical principles, to help educators create effective online learning communities. You can download and use it on any computer you have handy (including webhosts), yet it can scale from a single-teacher site to a University with 200,000 students." (...) "Moodle has a large and diverse user community with over 400,000 registered users on this site alone, speaking over 75 languages in 193 countries."