Khan Academy
Khan Academy is an online school specializing in K-12 math, and also featuring lessons in basic sciences and is now branching out into the humanities. The system is designed for students to work independently -- featuring a game-like platform for practicing mathematics.
Lessons are structured in a "knowledge web" -- building for basic arithmetic to algebra and beginning calculus. Videos are available to teach material, and data is collectable about a student's progress so a parent or teacher can help.