Brightstorm provides online study prep materials for the ACT and SAT tests. Although you must have a paid subscription for the ACT and SAT preparation materials, the site also includes some wonderful upper level mathematics tutorials that are totally FREE.
The mathematics tutorials are videos that include mathematics teachers explaining and demonstrating how to solve different types of mathematics problems. There are over 2,000 tutorial videos covering Algebra, Geometry, Trigonometry, Pre-Calculus and even Calculus. To watch the videos you do have to create an account but it's free and quick. There are even sample problems at the end of each video complete with answers.
This would be a fabulous site to share with students who might need some additional help at home while working on homework.