Bedtime Math
http://bedtimemath.org/
Bedtime Math is designed for parents who want to verbally pose fun math problems to their children. Each problem is differentiated to 3 levels (Wee Ones, Little Kids, and Big Kids) and all are intended to be figured out mentally (but of course using pencil and paper when needed is just fine!) There is an informational guide for parents as well as archived problems. The emphasis is on fun and doing math together in a relaxed way--- similar to the cozy experience of reading in bed together each night, except it's mathematics!
Nrich: Enriching Mathematics
http://nrich.maths.org/frontpage
This sight offers youngsters some great explorations and problems to ponder when they visit the student pages. There are 4 levels ranging from grades 1 through 12, and each provides a mix of games, problems to solve with paper & pencil, and a few online interactive activities and explorations. There is also an archive of past problems. NOTE: Elementary-aged children will need to work with a grown-up!