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Edit: some I have used or heard of that may or may not have .net support:
USB Bit Whacker - http://www.schmalzhaus.com/UBW/
Ultimarc control interfaces (haven't tried them) - http://www.ultimarc.com/
Tutorials, etc, here: https://learn.adafruit.com/adafruit-ft232h-breakout Includes some python code that works on Windows, Linux, etc.
A couple of sources:
http://www.ftdichip.com/Support/SoftwareExamples/MPSSE/FTCSP...
Used these for some simple automation needs in the past. Simple .net library available on the site. Not nearly as cheap as the one in the post (which I did have to google 3 different sites to find the price of)
Sometimes you do want to add GPIO to an existing Windows machine, but if you just want to mess around with .Net and some simple electronics, the parent's suggestion is cheap and simple (and e.g. https://github.com/raspberry-sharp/raspberry-sharp-io claims to support this use case.)