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/
