Hotmath.com provides homework help correlated with textbooks used from
middle-school math through calculus. Hotmath shows step-by-step explanations to
the odd-numbered problems (the ones where the numerical answer is typically in
the back of the book). Students (and parents!) can see how to do the
problems and then challenge themselves on the even-numbered problems. Hotmath
covers over 200 textbooks, and a one-years subscription is $49.
In addition to the step-by-step solutions, Hotmath has other resources (some
free) including graphing calculator tutorials, math videos, and even math