Maxim Digital Audio, otherwise know as MDA, has a large collection of free VST plug-ins for PC, amongst them being this Envelope plug-in. I downloaded MDA's free plug-ins in one swoop, but since it was a while ago I don't remember too much about the process. It shouldn't be a problem to download and install this plug-in as long as you have a properly compatible system, which means having a PC and a VST compatible DAW platform to run it on. It's got a plain and simple looking interface like most of the MDA plug-ins, with parameters for output, attack, release, and gain in the form of sliders. I don't believe that any manual was made for these free MDA plug-ins, but I could be wrong about this. Even if they do have one, I don't believe that it would be necessary.
I don't run this plug-in any longer ever since I made the switch from a PC to a Mac for recording, but when I did run the Maxim Digital Audio Envelope plug-in it was in Cubase SX, and of course on a PC. The PC was a Hewlett Packard Pavilion dv8000 lap top with a 3.0 Ghz processor and 2 GB of RAM. I definitely never had even the slightest issues running the plug-in even within this system, which is a lot less of a system from what I'm running now. It really takes up next to no processing power. I first used the plug-in about six years ago when I first started putting together my home studio and was experimenting with whatever free plug-ins I could get my hands on.
The Maxim Digital Audio Envelope plug-in definitely would never be a plug-in that I would have purchased, but for free it was worth having around anyone. I can't say that it saw all that much action for the time that I was running it, but perhaps in other hands it may have gotten used more often. It doesn't have all that realistic of a sound, but it really just depends on what type of sound you're going for and what type of projects you're working on. This is a rather old plug-in as far as plug-ins go, but for free I'd still recommend checking it out for yourself.