Moodle
Moodle is an online Virtual Learning Environment
As described on the main Moodle project site:
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 very active community involved in the further development of the software and thriving support forums. In addition to the core functionality of Moodle, including wikis, blogs, database, quizzes and learner management, there are a wide range of additional modules (plug ins) that can be uploaded and configured on the main system - for example OU are in the process of developing an e-portfolio for Moodle.
From the Open Source Schools presentation at BETT 2009, slides and commentary by Miles Berry, filmed by Hannah Wise of BBC Backstage, edited by Leon Cych of Learn4Life