Maths Champs http://mathschamps.co.uk This site has a great set of maths games that are sorted into different primary age groups. Practise multiplication, more/less than, decimals and much more. No sign in or registration needed to access the games.
RealTimeBoard
http://realtimeboard.com This is an amazing collaborative whiteboard where multiple users can edit a multimedia board in real time. The site allows you to signup and sign in using a Google account and you can access and add your files and media on your Google Docs/Drive area making this a fabulous companion to schools using Google Apps for education. You can write by typing or you can write in 'freehand' so you can use your interactive whiteboard to write and archive the lesson to use or refer to later.
Calculation Nation
http://calculationnation.nctm.org This is a superb maths site with flash games on perimeter, area, factors, fraction, multiples and many more. Play against the computer or challenge your friends. A free sign up is required.