There are three major open source products we are looking at. Moodle, Drupal (which was even used for the Whitehouse web site), and Claroline. Claroline is probably the simplest, but is also the least powerful. Drupal is probably the most powerful, but requires the most work. Moodle sort of fits in the middle of the two. It is very powerful, but very structured in it's own way. Just to give you an example, here is what we did to start setting up the class. Will not go through everything., but will give you a gist of how it works.
So we started by looking at what classes we have. Then we decided which class we wanted to look at.
You can then just pretty much plug the data right into the Moodle class setup.
From there you can go ahead and start setting up each weeks course materials. You can choose which week to edit in your syllabus.
Once you have chosen a week, you can add content from all kinds of sources. You obviously can enter you own content without reinventing the wheel.
Then once the class is set up, you can admire it on the main page with all the other classes. There is a lot more to it such as testing, student interaction, and a host of other possibilities.
http://www.claroline.net/ (in French)