Study Tools 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. 
math.stackexchange: this is a question/answer web site for anything math related.  No adds and it's totally FREE. 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).
purple math: Helping students gain understanding and self-confidence in algebra; includes lessons, quizzes, worksheets, homework guidelines and a study skills self survey; all FREE. free math practice tests and assessments; professionally created math problems in a variety of formats including comprehensive diagnostics, short concept-oriented practice tests, and concept-oriented flashcards.  All of the content is FREE.  They also have support for standardized tests such as the Accuplacer, ACT, LSAT, CLEP, GRE, GED, etc.