Csound

D. CSOUND AS A VST PLUGIN

Csound can be built into a VST or AU plugin through the use of the Csound host API. Refer to the section on using the Csound API for more details. 

If you are not well versed in low level computer programming you can just use Cabbage to create Csound based plugins.  See the section titled 'Cabbage' in Chapter 10.