Purplemath breaks down math topically into thoroughly explained modules (with little background knowledge required). If a student is missing fundamentals, this is an excellent resource to give them a quick refresher on a topic like logarithms or sequences and series. Most modules are broken into bite-sized parts and include practice problems with solutions.
How do I use purplemath?
Use the lesson index page to find the right topic and then lesson that is right for your. They topics are broken into:
Preliminary Topics
Beginning Algebra Topics
Intermediate Algebra Topics
Advanced Algebra Topics
Solving Word Problems
Appendix
…and the beginnings of Trig