Word Problems for Kids
http://www.mystfx.ca/special/mathproblems/welcome.html
Provides challenging problems by grade level from 5th – 12th grade.

Open Ended Math Problems
http://www.fi.edu/school/math2/index.html

Math Challenges for K-5 Learners
http://www.fi.edu/school/math2/index.html

Mathchallenge.net
http://www.mathschallenge.net

Figure This! Math Challenges for Families
http://www.figurethis.org/index.html

Math Playground
http://www.mathplayground.com/