Math.  How hard can it be?  It's just numbers, right?

Math Lessons

http://www.aaamath.com

This site has general math lessons and explanations searchable by both problem type and grade level.

Math Q&A

http://www.mathforum.org

Drexel University maintains this math help site.  The "Ask Dr. Math" section is very helpful for general math questions.

Math Theory

http://www.math-atlas.org

Let's hope we never have to use this.  Break the glass in case of student with questions about Multilinear Algebra and Matrix Theory.