I came across this simple music toy a few days ago and find I keep coming back to it for a quick break now and then.

It's easy to use. Select squares where you want it to play notes. If you play with stacking notes above and below in the same row, you can start to get a feel for how different chords are constructed (if you didn't already know).

For example, a basic chord is the "root" or "1", the "3rd", and the "5th". Pick a random square and call that the "1". Then, going up that line of squares, skip a square (the 2nd), select the "3rd", skip a square (the 4th), and select the "5th". There you have a basic chord. If you go on up and select the "8th" (8 up from the 1st one you selected), you will have a full octave. Try different things and you can quickly hear how different notes color a chord.

I think its neat how if even you have a bunch of notes playing, changing or adding just one, can really change the feel of the entire sequence. I also find it interesting what different geometric patterns sound like.

Anyway, I find it pretty neat to play with. You can check it out here:
http://lab.andre-michelle.com/tonematrix