Andyware has released Analog Box 2 (ABox2) for free - it previously sold for $69.95.
Analog Box is a low level, circuit based, modular software synthesizer that can also host VST plug-ins. You put sound objects on the screen, connect them together, listen and adjust. When you find something you like, you can record it to a wav file, or just save the circuit and play it again later.