1) American Mathematics Competitions (AMC)http://www.unl.edu/amc
The AMC seeks to increase interest in mathematics and to develop problem-solving ability through a series of friendly mathematics contests for junior and senior high school students.
2) Dynamic Worksheets: http://www.edhelper.com
Standardized test materials, mathematics puzzles, critical-thinking problems, word problems and dynamic worksheets.
3) MathMastery : http://www.mathmastery.com
MathMastery offers students, teachers, and parents online courses, daily mathematics problems, games, tips for learning at home, and classroom experiences designed to improve mathematics skills and increase understanding of mathematics concepts.
4) The MacTutor History of Mathematics Archive:
http://www-groups.dcs.st-and.ac.uk/~history/index.html
This is an extensive archive on the History of Mathematics. It includes biographies of over 1100 mathematicians, history topics, descriptions and histories of famous curves, and a 'Mathematicians of the Day' page. Many resources for integrating mathematics with history and writing.
5) Advanced Algebra topics: ... Still More Algebra Lessons Elsewhere ... Online questions covering sets, coordinates, algebra, trig and calculus from Maths Online http://whyslopes.com/lesson.html
6) MathNotes.com is a valuable on-line resource for both instructors and student users of the Lial/Hornsby/Miller Paperback Series. Teachers can find materials to enhance their courses, while students can strengthen their understanding through interactive tutorials and study aids, or, they can more deeply explore concepts through real-world applications and Web links for further research. http://www.mathnotes.com/index.html
7) http://www.a1tutor.com/tut_math.html#High%20School