Khan Academy: http://www.khanacademy.org/
This website offers great math videos explaining a variety of topics in detail. Click on "Browse our library" at the bottom of the page to search for videos on a specific topic.
Wolfram Alpha: http://www.wolframalpha.com/
This is a computational knowledge engine specifically for math. Think of it as a search engine for math problems. Especially useful for graphing and exploring problems.
Purple Math: http://www.purplemath.com/modules/index.htm
Purple Math offers lessons on a variety of basic, intermediate, and advanced algebra topics. Useful for reviewing the Algebra I skills necessary for Algebra II and Geometry.
Math Open Reference: http://www.mathopenref.com/index.html
Offers a lot of information, lessons, and example problems for all kinds of math. Most useful for Geometry and shows examples of proofs and constructions as well.