The Big Resources.
Wikipedia mathematics has many interesting articles, but doesn't gives decent explanations in many elementary topics.
MathWorld is a math encyclopedia with lots of articles, but not much of an explanation sometimes.
Cut The Knot has very nice Java applets, is VERY big for a personal site and a good reference! Many discussions and is pretty active. Only complaint is that sometimes its too formal missing many students.
Ask Dr. Math is a gigantic archive of questions/answers, you can ask questions (never did it though), and it sure has lots of nice entries! Excellent reference!
MAA columns are old and new columns of the Mathematica Association of America. Refreshing well written articles.
Mathpages is another big site, has lots of nice stuff, it is more advanced in general, it stopped growing at some point.
MacTutor History of mathematics has many good biographies of mathematicians, and nice articles on math history. It is also active.
Geometry Junkyard has lots of curiosities on geometry, and current research.
Math Atlas and PlanetMath are two less know math encyclopedias.
Smaller sites: Math is Fun is an educational site. Numericana question/answer style with some tricky ones. Inca Geometry has animations of geometry with some tough problems. Baez fun stuff has some nice articles from a known physicist. Art of problem solving is a community site with some different stuff. Words of mathematics is another historical reference.