ToneMatrix
It’s a grid. It has audio tones. The beat moves left to right. The tones move up and down. Click a box and it makes a sound. Fun, addictive, simple, and beautiful. Go play with it.
Here’s how Andre Michelle (the creator) describes ToneMatrix:
Simple sinewave synthesizer triggered by an ordinary 16step sequencer. Each triggered step causes a force on the underlaying wave-map, which makes it more cute.