thingstruments

  I've covered the quirky original ANS synthesizer before. This is the synthesizer used by Eduard Artemyev on soundracks for movies such as Solyaris and Stalker . A virtual ANS software synthesizer has been made, and best of all it is totally free!

Here's another free synth for you: Gimmick Graph - a synthesizer where you draw the waveform. This one is only available for the Mac users out there. Found via this Sonic State article .

Here's an open-source softsynth with awesome sounds: ZynAddSubFX . It is available for Linux, Windows and MacOS. Here's a sound demo with all sounds coming from ZynAddSubFX. The screenshot is rumored to be from an upcoming GUI rework...