Bespoke is a free (donaction possible) software modular synthesizer. It contains a bunch of modules, which you can connect together to create sounds.
The ANS synthesizer is a photoelectronic musical instrument created by Russian engineer Evgeny Murzin from 1937 to 1957. The technological basis of his invention was the method of graphical sound recording used in cinematography (developed in Russia concurrently with USA), which made it possible to obtain a visible image of a sound wave, as well as to realize the opposite goal—synthesizing a sound from an artificially drawn sound spectrogram. (more on Wikipedia) Get a free (donations possible) software version for iOS, Win, Mac from Alexander Zolotov.
Pianobook is a peer-to-peer community of composers, producers and sound smiths sharing their sounds for all to use for free. Set up by Spitfire Audio founder Christian Henson the aim of this community is promote the notion that the best sounds for music are made by people who make it! Here’s just one example, sounds of the Hohner Guitaret: