Moodle (acronym for Modular Object-Oriented Dynamic Learning Environment) is a free software e-learning platform, also known as a Learning Management System, or Virtual Learning Environment (VLE). As of June 2013 it had a user base of 83,008 registered and verified sites, serving 70,696,570 users in 7.5+ million courses with 1.2+ million teachers.
ISTL's choice for MOODLE is justified by the fact that it is easy to learn, it is free to use (except hosting costs), it is highly flexible and easy to integrate with other services.