MOODLE
Moodle is a popular Course Managment System that is OpenSource (free to download, use and alter). It is used on thousands of sites world-wide, including our courses site, Search Engine College.
It is a powerful system (if not the most aesthetically pleasing) that can be downloaded here.
For a blog post regarding what to look for in selecting a new course managment system, see What To Look For In A Course (or Content) Management System.
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CategorySoftware