TopmarksLots of interactive games on all the maths topics. Make sure you look at the games under the 7-11 years categories for the right challenge for Year 3/4.
BBC BitesizeThe old BBC Bitesize website with maths information, games and quizzes on all areas of maths
BBC Bitesize (NEW)This is home of the new BBC BItesize, which is still being put together. It does, however, already have a large selection of videos on plenty of maths areas already.
Every SchoolA website full of games on every maths topic. Look down the left side of the page to find the topics; you want the ones listed under 'Key Stage 2'
Maths FrameMore free games to play on a variety of maths topics. If you browse this site, you'll also find that it lists all the objectives for the new curriculum and suggests games that will help children achieve that objective.
Woodlands JuniorA website of a school in Kent who have gathered together lots of resources to do with all areas of maths. Everything is organised under topic.