Spectrogonk is a fun and easy-to-read musical spectrum analyzer.
You read it like a clock-face with C at 12 o'clock, E at 4 o'clock, and so on. Low frequencies are near the center of the clock, high ones out at the edge.
Sing into it and it'll show you what harmonics you've got going on in your voice
Adds a whole new dimension to synth nob-twiddling
Turn on the gridlines and it makes a handy chromatic tuner
Play it some electronic music for a neat lightshow
Music credit: 'Time for the walk of the day' by Komiku