Csound is a sound and music synthesis system. Drawing from over 450 signal processing modules, it can be used to model virtually any synthesizer or multi-effect processor. It can work either in real-time or as a compiler.
Csound is to sound synthesis as C is to programming.
First: I'm a big fan of Csound. Drawback: in the actual version the adsyn produces only noise. An old (4.08) Windows version works fine via wine though, but it is very inconvenient and a slow workflow.