I's very old (1991), but the concept behind it has never been developed much further during the past twenty years so I consider it still interesting.
It allows you to paint a sonogram using specialized 'brushes' and then synthesize the corresponding audio waveform. [...] The main added features in RGS are that there are now 256 levels of amplitude to choose from and the ability to synthesize while you are drawing in nearly real time.
When I was composing and experimenting merely with an Amiga this program was a big discover for industrial sounds as it produces strange noises and drones. Spectral analysis is a powerful method for discovering underlying structures in sounds and being able to draw your own is truly stimulating. However, even writing your own name or randomly draw harmonics can result in very interesting sounds.
RGS can be run on a real or emulated Amiga and can be found here and documentation here.
As usual, I made a collection of .WAVs samples of me exploring the program: