When there is something I need to know or review, I find one of the following websites usually has the answers.
The Khan Academy has an amazing collection of over 1000 videos on YouTube. Topics cover everything from basic arithmetic to physics, calculus, and finance. It is a not-for-profit organization whose mission is to provide high quality education for everyone.
Wolfram|Alpha allows you to input words or equations and get solutions. They explain it this way "We aim to collect and curate all objective data; implement every known model, method, and algorithm; and make it possible to compute whatever can be computed about anything." It's a little like a search engine, a little like a calculator, and full of information.
MIT OpenCourseWare contains almost all MIT course content. It is open and available to everyone and contains free lecture notes, exams, and videos from MIT.
Brightstorm has videos on math topics ranging from Algebra to Calculus. Over 2000 free online videos are presented by some of the best educators. There are also lessons for SAT and ACT Prep, but after a free 7-day trial, there is a charge for this.