From Andrew Allenson's Pickledonion comes this simple and elegant Shockwave sound toy that is delightfully mesmerizing and relaxing to play with.
Phase Toy is a simple sequencer that allows you to create compositions using a point-and-click interface. Just click on a cell to have the sequencer play the corresponding tone when the 'play head' sweeps over it. Click and drag to add several tones at once.
Once familiar with adding tones, experiment with adding (or removing) divisions to each section by clicking the plus (+) or minus (-) sign. Doing so effectively changes the number of tones that are played before repeating the sequence again. You can have as few as one (1) and as many as 16 in each section. Notice that the play head in the first section plays at a slower speed than the other two. This causes some interesting phasing to occur as explained by Parabasis in the comments.
Unfortunately, there is no easy way to save or share compositions, but the source code is available should you feel ambitious enough to add that functionality to it. And yet even in its present form, Phase Toy still makes a nice little diversion. After all, we can all use some music to play with every now and then. =)