My guess is you're thinking of no$gmb, a very early Game Boy emulator:

https://gbatemp.net/threads/no-gmb-2-5-dos-full-version.6039...

I've got fond memories of using this to get a preview of Pokemon Gold before it was released in NA!

Found it

MetalNES, transistor level NES emulation https://github.com/iaddis/metalnes

While searching I also found a new one, VisualNES https://kaiokendev.github.io/nes/about

There is also one for GB https://github.com/aappleby/MetroBoy