Starting with a 100 Grid, the first player chooses a positive even number that is less than 50, and crosses it out on the grid. The second player chooses a number to cross out. The number must be a factor or multiple of the first number.

Players continue to take it in turns to cross out numbers, at each stage choosing a number that is a factor or multiple of the number just crossed out by the other player. The first person who is unable to cross out a number loses.

http://nrich.maths.org/public/viewer.php?obj_id=5468

This is a fun game from Ambleside Primary. Choose your level. Add the numbers in the bottom row to get numbers in the middle row. Then add the numbers in the middle row to get the answer at the top of the pyramid. Now check your answer. There are three levels - easy, medium and hard.
http://www.amblesideprimary.com/ambleweb/mentalmaths/pyramid.html

Line up the people in order of height. Then find the median, Q1 and Q3. For some reason they all look very cranky...
http://www.subtangent.com/maths/median-and-quartiles.php

A flash program that displays a large clock that keeps time via the system clock. A very handy utility for both teaching about time, and as a classroom management tool.

Web address:
http://www.wmnet.org.uk/wmnet/custom/files_uploaded/uploaded_resources/5...

Right click and choose "Save Link As ..." if you want to download the flash program.