Reaktor Blocks is Native Instruments' virtual modular synth environment. It's effectively a friendlier layer of Reaktor 6 ($199.00) with very hardware-like user interfaces on the modules. It's also open to modules from third-party developers.
The AM107VC from Amazing Machines is a "voltage-controlled" envelope generator with an AHDSR envelope profile: attack, hold, decay, sustain, and release. Each parameter, plus an overall curve setting, can receive a different virtual control voltage from elsewhere in your NI Blocks configuration. It can cycle or fire in one-shot fashion, or be triggered manually. Each stage can last up to 100 seconds for use in subtly evolving and generative patches. Check the demo video below for a knob-by-knob overview:
Best of all, it costs $12.00 US. Yup. Twelve bucks, the price of a pork belly on brioche breakfast sandwich at a beardy gastropub. Only the AM107 will only fatten up your sound, not your arteries. Through July 31, other select virtual modules and hardware products from Amazing Machines are up to 33% off as well. More info here.