Ample Design have put this fascinating little gadget called Visual Acoustics on their website. [Biretta-doff: Jay is Games]
I suspect my more musical readers will be able to make much more harmonious noises with this tool than I can. Basically, one draws on the screen with one's mouse in order to play various musical instruments. Playing around with the volume and delay settings creates some interesting effects. I'd advise beginning with the piano only to get a feel for the interface and the settings, before moving on to paying several instruments simultaneously.