In 2009, the state of Virginia had a contest called the Virginia Mobile Learning Apps Development Challenge. This contest challenged developers to create apps for middle school mathematics students. The winner of the contest was a doctoral student named Todd Bowden and his creation is a terrific free (I like free!) applet named Numberline.
Numberline asks players to place decimals, fractions, and percentiles on a number line. Each math figure is contained in a bubble and you drag the bubble to the line. You receive points based on how quickly and correctly you can place the figures.
This is a great way to learn the relationships between decimals, fractions, and percentiles. It can be used in a small group for targeted math instruction.