Excellent range of games and interactives http://www.woodlands-junior.kent.sch.uk/maths/index.html
Instant Maths games KS 1/2 is for up to Year 5 and KS 3/4 is from Year 6 to 10 in Australia - http://www.mathszone.co.uk/
Learning and Teaching Scotland - Searchable maths games by year level - http://www.ltscotland.org.uk/5to14/resources/mathsgames/index.asp
Maths games from Learning Today blog - http://www.learningtoday.com/corporate/maths-games.asp
Practising Division - http://www.amblesideprimary.com/ambleweb/mentalmaths/dividermachine.html
Cyberchase - games based on the TV cartoon - http://pbskids.org/cyberchase/allgames.html