I know this thread is old now, and some things have changed, but there are so many great links and reviews it is still a very useful resource. I hope people will keep updating. I would like to add a few that are new options or were not listed originally. Many of these will be posting their class offerings for summer/fall in just a few weeks but I don't think most have them up yet.
(Interface is very easy to use once you are signed up for a class.)
MathCloud (self-paced but can be linked with a teacher)
Open Tent Academy