dplug is a library for audio plugin development. https://github.com/p0nce/dplug http://code.dlang.org/packages/dplug
It's aim is to be a lean alternative to JUCE and IPlug, the most used C++ libraries in this space.
It is currently less useful since supporting only VST 2.x on Windows. The plan is to gradually add more formats and OS support (VST Mac and AudioUnit should be first).
Because of the particular need of audio plugins (lazy screen updates, message dispatch made by host, threading, child window...), dplug has its own windowing. It then use a deffered software renderer to have a nice procedural UI and keep installer sizes low.