The Little Enveloper is a free Max for Live : it adds a rhythmic envelope to the input sound.
You can use it to create timbre/rhythmic variations on your sound. Using it in Audio Racks can transform a fixed sound in a complex polyrhythmic structure.
What kind of plug-in is it?
It is a Max for Live device: to run The Little Enveloper you need Live 9 and Max for Live.
Is it Mac/Windows compatible?
Yes, it is compatible with both Mac and Windows (provided you have Ableton Live and Max for Live installed on your computer).
How much does it cost?
Here is a video of the device: