http://www.khanacademy.com With a library of over 3,500 videos on everything from arithmetic to physics, finance, and history and hundreds of skills to practice, to help your learn what you want, when you want, at your own pace.
The World of Math Online
http://www.math.com Free math homework help, math tutoring, math formulas and cool math stuff. Recommendations on books, calculators and products for students, parents, and teachers.
http://www.sosmath.com is your free resource for math review material from Algebra to Differential Equations!
http://www.coolmath4kids.com Cool Math has free online cool math lessons, cool math games and fun math activities. Really clear math lessons (pre-algebra, algebra, precalculus), cool math games
http://math2.org/index.html Source for all your math needs- geometry, algebra, trigonometry, calculus, and more.
http://archives.math.utk.edu/topics A page of the Mathematics Archives - Links organized by topic .
The Math Forum @ Drexel
http://mathforum.org/library The Math Forum is the comprehensive resource for math education on the Internet. Some features include a K-12 math expert help service, an extensive database of math concepts.
Weights and Measures
http://www.refdesk.com/factmeas.html Weights and measure resources
Math World Problems
http://www.mathstories.com Over 2000 math word problems for students to learn from and enjoy. The pages are sorted by topic and level of difficulty. Each problem is designed to improve elementary and middle school students' critical thinking and problem-solving skills.