Probably the most visual ADSR in the world

The ADSR boasts two ADSR and multiple functions such as Loop, EndOf, CV control and internal routing. Built around an Arduino the MiaW ADSR is a DIY module. With intenuverters on the output you can tame the signal between -8 to +8V and produce both positive and negative envelopes.
The main "feature" would be its visual aspect as it shows the envelope in all its four stages and thereby shows in a beautiful way how the envelope stages affect the output.

Video describing the module and its features:
https://youtu.be/jFjOcIDxjK0

https://www.tindie.com/products/Sourcery/adsr/