One for the techies:

Home of the open-ish version of Canvas, the LMS that is used by canvas.net for its MOOCs. Though not as feature-complete as Moodle, it is an elegant system designed to scale, with better social tools and a much smarter and slicker interface. Warning - it is not fun to try to install this yourself unless you really like grappling with software installations. A lot of dependencies and libraries and components that must be configured.