I'm teaching an Intro to Proofs course this Fall, and am planning to have students also sign up online for a course called Introduction to Mathematical thinking, a free massively open online course (MOOC) offered by coursera . The course will be delivered by Stanford professor Keith Devlin, and looking at the online course outline, it looks like a perfect supplement for my transitions course.
This will be the first time I am trying this, and I myself have not taken any coursera course. Looking forward to this experiment. It can only enhance the students' learning experience, and it doesn't cost anything.