www.ai-class.com
Taught by professors Sebastian Thrun and Peter Norvig as part of an experiment to see how vast an audience they could reach. 169,000 people have confirmed their registration (initial reg. figures were higher). I don't know if you can still register, but all class materials appear to be accessible regardless (streaming video, so a reliable high-speed connection is recommended).
There are two levels: basic and advanced, sharing the same lectures. Advanced students will have homework and midterms; basic students will be quizzed on their knowledge. Note that this is a university-level class.