I waste a lot of time on the internet, but it’s usually wasted on reading, not on playing with the myriad of interactive gizmos that are available. This one, though, is most excellent: the ToneMatrix. To use it, just click on squares in the matrix to turn them on or off, and see what happens. You can also draw a pattern of squares by holding down the mouse button and dragging the cursor around.
The idea is based on another gizmo, the Tenori-On from Yamaha, which is a synthesizer with an ingenious interface.
Here is a video showing how the ToneMatrix is used as part of a more complex computer program called AudioTool, used to create synthesized music.