Math Playground (K - 6) - educational math activities for students. 

AAA Math (K - 8) - online math materials for students in grades K - 8.

Cool Math 4 Kids: "The Amusement Park of Math and More" includes "Lemonade Stand", which is a challenging simulation where students have full control of decision-making, as well as dozens of Brain benders to exercise math skills with topics ranging from alphabet soup to geometry and math games to challenge students preschool to adult.

Illuminations : Principles and Standards for School Mathematics  is a partner in the Marco Polo Project. Based on NCTM (National Council of Teachers of Mathematics) standards, this online resource offers rich, comprehensive math content in five grade level groups from K-12.

Funbrain: The #1 site for online educational games for kids of all ages. 

Disney Family offers free online educational elementary games for kids as well as printable activities.

Fractions Calculator - this helpful calculator adds, subtracts, multiplies, divides and simplifies fractions.  The solution shows how the problem was solved to get the correct answer.