Woodlands Maths Zone provides several fun online interactive math activities aimed at 7-11 year olds.

Nrich is a great maths resource for stretching the brain with lots of puzzles and challenges.
Funbrain Math Arcade provides fun math games and activities.
Big Brain - the Cure for Multiplication Problems. Base version is available for free.

Operation - a challenging maths game for older children

Mr Nussbaum - dozens of games and other activities for children from Kindergarten to Year 8

Big Brainz Multiplication Games - Teaches multiplication table mastery