GRNLR is an open source granular synthesis VST/AU instrument for Mac and Windows with a focus on flexibility and ease of use. It offers a approach to granular synthesis that allows for a great number of simultaneous playing grains as well as supporting extremely short and long grains. The instrument comes with a tutorial documenting every stage of the development process. The tutorial is intended as a resource for people wanting to learn how to write their own instruments using the JUCE Framework.
Here is a playlist of 7 short pieces I wrote using the plugin.