math.com: has good overview explanations for pre-algebra through calculus concepts and terms. It also has a section that gives study advice as well as strategies to overcome test anxiety. However, there is no step by step help available on this site.
mathway.com: is easily navigated and has several options for what a student might be looking for from a given equation (solve for x, solve for y, find the 0, etc). The site costs $19.99/month. For no fee, it does show the answer to a given equation and it can also generate sample worksheets for any subject type (like arithmetic operations).