GoMath(site info)
Online math tutoring service designed primarily for preparation for the SAT. Includes e-mail help within 24 hours, interactive quizzes and games, and help organized by topic.
http://www.gomath.com/
4.
Hotmath(site info)
Solutions problems in popular US textbooks, algebra through calculus. Solutions include hints and explained steps up to and including the final answer.
http://www.hotmath.org