I could see something like that being useful. Would be nice if you could select a category for "what kind of question" do you want. I'd also like to see immediate feedback after each question, including an explanation of how to solve the question properly (if the wrong answer is submitted).

I could see paying for a service like that. Especially if the scope of the material ranged all the way from high-school algebra through - at least - college level calc, diff eq, linear algebra, etc. I say this as somebody who's just now attempting to go back through his old college algebra/calc notes, refresh on calc, and then trying to teach himself differential equations and linear algebra. For somebody like me, a site like this could be really, really useful.