Wolfram Alpha

Powerful database knowledge engine that can answer all sorts of questions but also will show everything you wanted to know about a function.

CK-12

An online textbook resource that allows you to create your own text by using and editing their open source books. Great for increasing math literacy!

Kahn Academy

A comprehensive resource with videos and practice for all levels of math. Their SAT prep is also free and awesome!

IXL
Interactive adaptive learning program that has all math levels K-12. Provides practice for individual skills and allows for targeted remediation. Requires paid account – but worth it.