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 50,000-student University.
Moodle has a large and diverse user community with over 100,000 registered users on this site alone, speaking over 70 languages in over 150 countries. The best place to start is Using Moodle, which is where the main international discussions are held in English, but it has a variety of groups discussing other topics and in other languages.
Check out the Moodle Demonstration Courses or read the latest Moodle Buzz. Moodle is available in a variety of download packages with different levels of stability, as well as via CVS from Sourceforge.net. A number of additional modules/plugins and language packs are also available.