From Researching Virtual Initiatives in Education
Moodle - abbreviation for Modular Object-Oriented Dynamic Learning Environment - is a free and open-source Virtual learning environment.
As of January 2010 it had a user base of 45,721 registered and verified sites, serving 32 million users in 3 million courses.
Moodle is developed by Martin Dougiamas to help educators create online courses with a focus on interaction and collaborative construction of content.
Moodle Pty Ltd (based in Perth, Western Australia) does most of the development of Moodle.
Moodle's open source license and modular design allow any developer to create additional modules and features
For more details see http://moodle.org
Sites using Moodle
Open Source alternatives to Moodle
See the interesting post at http://barrysampson.com/2009/04/open-source-lms-10-alternatives-to-moodle/