Analog Box is an an open-source, 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. If you are looking for unique sounds that no one else has this software is a goldmine. This style of modular programming will teach you much about synthesis, while giving you ton's of new sounds for your compositions. I highly recommend Abox.
Online Help is here.
When you grab Abox make sure to download some of the user built files to check out what can be done and jump start your experiments. Here.
Windows 32bit .... Works fine in Win7 64bit. Unfortunately there is no Mac version.