Fractionizer from Center for Game Science enables you to visualise a fantastic array of fractions. It is a versatile teaching tool that works well on an interactive whiteboard as it is possible to visualise fractions in strip, grid, pie and numberline formats.
You can compare, add, subtract and multiply two fractions together with clever animations. With addition and subtraction the animations demonstrate the creation of common denominators.
It has the usual controls for animations so you can pause them at any time to discuss what is happening. We particularly like the fact the animations are viewable in different formats as this can help to consolidate the concepts.
The only disadvantage to Fractionizer is that is only available for desktop machines as it is a Flash resource.