Khan Academy: Growing in popularity around the world, the Khan Academy is full of helpful videos explaining all sorts of math topics (they have videos on other school subjects too).
Art of Problem Solving: This math website has lots of stuff including math discussion forums, math competition information, and classes students can enroll into… but we especially like their clear videos!
Algebra.Help: As soon as you open this page, you see all the topics you can find lessons, examples, and worksheets about. No need to dig deep into the website to figure out what it actually covers.
PatrickJMT: This website has clear math videos on tons of topics.
Math Is Fun: The main content of the site is aimed at basic math skills. However you will find some more complex stuff, and some easier bits. Hopefully there should be something for everybody.
Math Goodies: site to learn pre-algebra, as the site focuses on theory and information, but also provides educational exercises immediately following the lesson.