The Steinberg VST API is supported by virtually all professional audio applications on Windows, Linux and Mac OSX...
Opaz PlugDK is an experiment that allows you to develop VST Plug-ins using the Ruby language (via JRuby)...The plugin reloads instantly when a file is changed, e.g. code changes are audible immediately.
It also comes with an IRB console ... [and] wraps the VST API in a very nice DSL.
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.
Opaz PlugDK Demo