Incredibox is a online music arranging and remixing tool that looks great, is easy to use and is free. It also happens to work really well on an interactive whiteboard.
The concept is simple: drag an icon to one of the “dudes” (as I like to call them!) on the screen and he’ll start beatboxing, singing or whistling. You can use 7 dudes at once and with a total of 20 sounds in 5 different categories to choose from, there’s a wide range of combinations possible.