Fun number based games- particularly popular with the boys in my class. games work on division, addition, multiplication and subtraction of simple numbers. When you select a game, make sure select- the create game tab- which will just take you to a new game instead of an existing one.
FANTASTIC SITE for maths games - HEAPS of interactive maths games. Stop the Clock is great for time (varying levels), Power Lines is great for mathematical reasoning
This site is filled with loads of challenges which will engage students of all ages. In each game their is a problem to solve, and children are timed on how quickly they can do it. All games are fully interactive and don't take long to load. Just be careful which you pick as some are quick tricky. More suited to upper school.
In this game from Count On you practise finding a fraction of a number. You can play it as a two-player game or a single player can play against the computer.
A good game for negative numbers and mathematical thinking. This game is for two players - you can use the interactivity below, or you could draw a number line on a piece of paper and find a counter to use. In both cases, you will need two dice. Decide who is Positive and who is Negative. Positive moves from left to right and Negative moves from right to left. Take it in turns to throw the two dice and add the scores then move that number of places in your direction. If the counter reaches −13, Negative has won. If the counter reaches 13, Positive has won.