Analog Box is a low level, circuit based, modular software synthesizer. 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.

http://www.andyware.com/abox2/index.html

Downloads:

ABox2Demo (not really a demo, it's freeware) and ABox2Help:
http://www.andyware.com/abox2/download/abox2demo.zip
ABox235free only, no help files, faster download:
http://www.andyware.com/abox2/download/abox235demo.zip
ABox2Help only:
http://www.andyware.com/abox2/download/abox2help.zip