WebMath has been around for over a decade and it is a site that contains explanations and tools for learning and practicing dozens of math-related exercises, from simple functions to complex calculus. Composed of forms to fill-in it returns analysis of a problem and in many cases provides a step-by-step solution. This can support students in understanding math processes.
The only downside is that kids could potentially put in problems and get their answers (and work shown) without actually doing the legwork.
Parent or teacher involvement is key when kids use this site for homework.