Moodle

This course management software is open source, which means that anyone can obtain its source code and use (as well as modify it to suit their needs). In short, there are no fees for its use. It runs on a variety of operating systems (Linux, Windows XP, Mac OS X and Netware 6) and also requires Apache, PHP, and MySQL or PostgreSQL. It is very much an international effort as it is available in nearly 40 languages. It is very full-featured, and an active community contributes to it.

http://moodle.org/