New Zealand Maths - Ministry of Education site with a wide range of fun activities and teaching resources.

Two Minute Warning - basic facts against the clock!

AddEmUp - Remove matching sums from the grid.

MathFactsPro - Addition, Subtraction, Multiplication and Division, age-graded and with printable results.

MathMagician - Basic Facts, single and mixed operations, timed and scored.

Moon Maths - Multiplication with Little Green Men!

Sheppard Software- Basic Facts, plus loads of games on other maths-related themes

KCC Teachers Basic Facts - Links to a range of Basic Facts games.

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