Creating your own Plug-in
Audacity is designed to support plug-in modules that extend its functionality.
There is a built in XLisp interpreter that is used mainly for creating new sound effects. It can also be used to create labels. It's mature, stable, and has been in Audacity from the start. Nyquist. Only limited documentation is available.
Limited ability to do 'the same thing' to a large number of files. Originally written for cleaning up lots of audio tapes, applying noise removal, removing long silences too and converting to mp3. Stable, but quite limited in what it can do.
Still very very experimental. Under windows you can use ActivePerl to drive Audacity. To use this (August 2007) you should be an experienced C++ developer too, as you will almost certainly need to add code into Audacity to process commands which are not yet handled.
Sound effects. C or C++, Ladspa Plug-in. GUI is built from simple instructions.
VST Plug-in. GUI is built by the plug in directly.
Work in progress. C or C++. GUI is built from simple instructions. Similar in concept to LADSPA except this it is designed particularly for analysis of sound, so for example finding particular words in a spoken text.
Plug-In that takes over the entire GUI of Audacity and uses the underlying services for its own ends. (not to be confused with 'Jack')
Work in progress. A still experimental method for adding any feature to Audacity as a plug in. The plug in can use any feature of wxWidgets, and has an API for interaction with the Audacity program.