If you are reading this page, and have some time, I would appreciate help in organizing this page into useful categories! Thanks.
Recommended by a parent: http://ca.ixl.com/ 
It is based on the Ontario Curriculum however the lessons are applicable anywhere.
I checked out the Practice section and it's got a great approach.
Highly recommended by a favourite blogger, http://dragonboxapp.com/ is an app that teaches algebra to children 5+ or 12+ ages, using a gaming format.
- http://www.tenmarks.com/index.html - assessment aligned to common core
- http://plus.maths.org/content/ Free online publication highlighting the practical applications of mathematics. Includes mathematics-related news articles, podcasts, and mathematics puzzles designed around "real-life" scenarios.
- MathGrade: everyday math as podcasts
- Dan's MathCast: more math podcasts
- STEM Resources - Science, Technology, Engineering & Math) Education Resources
- http://www.syvum.com/math/ - quizzes (also http://www.syvum.com/teasers/)
- and http://www.syvum.com/math/trigo/ (trigonometry)
- http://teachingcollegemath.com/?p=1898 - good advice for high school, too
- HotChalk - http://www.lessonplanspage.com/Math.htm
- UCCP Open Access Lessons: http://www.ucopenaccess.org/
- http://www.hippocampus.org/
- Create a Graph
- StudyJams
- Univ of Tennessee Math Archives
- Free Lectures through video: http://www.khanacademy.org/
- Short how-to videos at the educational site, Next Vista
- http://adifference.blogspot.com/
- http://calculationnation.nctm.org/ - recommended by Free Tech for Teachers
- http://illuminations.nctm.org/
- http://www.mathpickle.com - pose 1,000,000 questions to your math students, videos explaining math principles
- http://nlvm.usu.edu/en/nav/topic_t_1.html - a collection of manipulatives in digital format
- http://nrich.maths.org/frontpage
- Calculus on the Web An internet tutoring utility for learning and practicing calculus
- Visual Calculus Short descriptions and examples for limits, derivatives, and integrals. Various plug-ins are needed to view some of the pages
- Math Teaching Resources Java programs, flash movies and interactive templates
- National Library of Virtual Manipulatives Virtual manipulatives for various mathematical topics
- Interactive Mathematics Project Various Flash activities demonstrating math concepts
- The Connected Curriculum Project Interactive learning materials for math and its applications
- Java Script math applets| Middle School | Trigonometry | Calculus | Vector | Complex number | Conics |
- Equation Match Find matching pairs of equations by solving for x in each equation
- Math Open Reference Using interactive tools and compelling animations, it provides an engaging way to learn and explore the subject
- E-examples for Math Standards aligned lessons from Illuminations, a rich math resource from NCTM
- Shodor Interactive creation, collection, evaluation and dissemination of interactive Java-based courseware for exploration in science and mathematics
- S.O.S. Mathematics - Algebra - fractions, units of conversion, complex numbers, quadratic equations, solving equations, systems of equations, inequalities and others. Explanations are step-by-step and include examples. Some practice questions are given.
- Technical Tutoring Math Help - contains links to various mathematics topics ranging from basic math operations to calculus. Use this website to review number theory, exponents, linear equations, factoring polynomials, operations with radicals, polynomial equations, rational expressions, linear inequalities and other topics.
- The Math Page - covers skills in basic arithmetic, algebra, including algebraic expressions, linear equations, quadratic equations, inequalities, absolute value, word problems, exponents, factoring, radicals, complex numbers and many others.
- Understanding Algebra - an on-line textbook covering topics and skills in arithmetic, pre-algebra and algebra. It also offers helpful strategies for solving word problems.
- Purplemath - variety of topics including fractions, absolute value, percents, ratio and proportion, distance formula, linear inequalities, radicals, factoring quadratics, exponents and many others.