Interactive Math Miscellany and Puzzles: This is a wonderful site. It contains lots of neat math facts and can be quite challenging. More for the teacher, but older kids would love exploring. A well designed site.

MathsNet Puzzles: Perhaps a little difficult, but they vary. Teachers can pick and choose appropriate ones for this age group.

Mathstories.com: The goal of this web site is to help grade school children improve their math problem-solving skills. This site started out as only a few pages, but it has grown to hundreds of pages with over 2,000 math word problems for children to enjoy!