I would have loved this as an undergrad. I used to go to the library all the time for the problems in other textbooks.
You need Wolfram Pro, however.
Very cool. I enjoy Khan Academy's practice problems for students but unfortunately they stop at the very beginning of calculus.
The interactivity of a problem set can really help motivate some students so I hope to see more of this!
Edit: Well I wrote too fast apparently. They seem to limit their calculus problems to derivatives. Where are the problem sets for differentiation for common applications for "calculus for fun"?