I'm pretty happy with the Crypto class Coursera is running right now. The lectures are pretty info-dense and the homework promotes at least some thinking.

I think it's better than the AI class I did the first go around.

The best of these classes for me so far, though, is the MITx class on circuits and electronics. The professor is fantastic- great at explaining things and obviously having fun teaching the class. The homework is challenging, at least for me, and I feel like I'm learning a huge amount. Very very pleased with that one.