While tis company has several games that have free versions, this one has captured the interest of several in the house - from the kiddo we actually bought it for to his dad. While it is created for elementary age, it's a great way to quickly visualize fractions, decimals, negative numbers, etc. In relation to other numbers. Well worth the $2 we spent.

http://itunes.apple.com/us/app/motion-math/id392489333?mt=8