These websites offer many interactive tutorials, detailed explanatory pages, and videos to provide free online tutoring in a variety of subjects.

Multiple Subjects:
    HippoCampus - Multimedia lessons and course materials for twelve subjects, as well as interactive aids for over 100 textbooks.
    CliffsNotes - The popular literature guides, as well as pages on language, science, and other subjects.

Math:
    Interactive Mathematics - High school math, algebra, probability, calculus, and other high-end mathematical topics.     S.O.S. Mathematics - Over 2500 pages explaining algebra, trigonometry, calculus, differential equations, and other topics.
    Algebra.help - A collection of lessons, online calculators, and worksheets for students learning algebra.
    Quadratic Equation Solver - A calculator which can generate solutions to quadratic equations.
    Wolfram MathWorld - An extensive resource for math subjects from algebra to calculus and beyond, from the makers of Mathematica.

English:
    The Purdue Online Writing Lab (OWL) - Over 200 free lessons and guides for grades 7 through 12.
    Guide to Grammar and Writing - A site explaining the mechanics of writing with many detailed pages.

Science:
    The Physics Classroom - Multimedia physics tutorials with many videos and animations for high school level physics.