The Math Forum is perhaps the most extensive math education site on the web.
Its high school section includes problems of the week,
the popular Ask Dr. Math, and more.
Alexander Bogomolny has written a remarkably extensive collection of web pages
on various mathematics topics. He has a special interest in
mathematical games or puzzles,
but there are also sections on
algebra,
geometry,
probability, and more.
Although described as a list of "teaching materials,"
this large collection of links includes many sites useful to students.
Note especially the Topics in Mathematics
heading, which leads to whole pages of links on
algebra,
geometry,
pre-calculus,
calculus,
statistics,
and more.
The Geometer's Sketchpad exploratory software is used in many geometry classes at LHS.
This site from the software publisher includes sample sketches and project ideas, and a
demo version that you can download
if you don't own the software.
This substantial work is a compilation of the geometric knowledge of the ancient Greeks.
Start with the "Book I" link. You can choose to read in English or Greek.