AlgebraHelp
This site offers tons of explanations of difficult topics in algebra.
Purple Math
Purple Math also helps students with their algebra questions. It is useful, also because it provides a forum for students and parents can communicate with each other about similar algebraic topics.
WolframAlpha
Wolfram Alpha is an amazing site that I used during my undergrad. It is a search engine; just type in your topic or question and it will give you everything you need to know!