The Steinberg VST API is supported by virtually all professional audio applications on Windows, Linux and Mac OSX. It is a de facto standard that allows to develop audio effects such as a delay and virtual instruments like pianos or guitars in form of pure software.
Opaz PlugDK is an experiment that allows to develop VST Plug-ins using the Ruby language (via JRuby). It comes with very nice features bringing the "scripting" approach to VST development. The plugin reloads instantly when a file is changed, e.g. code changes are audible immediately. Absolutely no compile-package-test cycles are necessary!
It also comes with an IRB console that allows you to attach to a running Ruby plug-in, e.g. to script the plugin live, while it is running (very much like using Ruby as a macro language for the plug-in). Moreover, it wraps the VST API in a very nice DSL, making the code much cleaner and understandable. For increased performance, it is very easy to mix-and-match Java and Ruby code. It also comes with Rake tasks that bundle your plugin for windows, mac and linux righ-away.
BTW: Sorry for the stuttering audio track. The screen-recording software ate most of the cpu cycles on the machine I was using to record.
Further info: http://blog.logeek.fr/2009/11/17/how-to-prototype-vst-audio-plugins-with-jruby-and-java
Project website: http://github.com/thbar/opaz-plugdk
jVSTwRapper website: http://jvstwrapper.sourceforge.net
JRuby website: http://jruby.org
The music loop I used is called "loop022" from freesound.org user bebeto: http://www.freesound.org/samplesViewSingle.php?id=24039