Have you ever had contact with these thingamajigs? If so, are they important/special for you in any way?

They sure are for me. My first contact with the NES (and retrogaming in general) was through a 99-in-1 plug-'n-play controller that looked like the Genesis' except it was blue and silver with three yellow buttons and it had the mandatory turbo trigger. While most of its library was composed of the "classic" multicart mainstays, it also had Contra without the logo and with a level select screen built-in... labeled as a Hello Kitty game, IIRC

This was waaaay back in 2005/6, though. More recently, I've acquired a Tectoy/@Games Master System/Game Gear plug-'n-play device for my younger brother (who's actually pretty into retrogames too), and boy is it a peculiar one. First off, the controller has (besides the usual start and reset buttons and on/off switch) three buttons labeled A, B and C as opposed to two labeled 1 and 2 (I suppose it's the same framework used for a Genesis version), the PCM emulation on it is crap and its library of 30 games contain quite a few Game Gear exclusives and the Model 1 Master System BIOS ROM, with Snail Maze included.

While I was playing The Ninja on it, I accidentally held the reset/menu button for more than one second and the result was the discovery of a hidden, undocumented test screen! It's a really simple sound/image/button test, but I found it cool nonetheless. Besides, it helped me find out that the buttons are getting kinda faulty and that my TV's AV sound channels are actually reversed - red plug goes in white port, white port goes in red plug. This isn't apparent with my N64, though, so I guess it's one of the device's many quirks?

But anyway, have you guys ever experienced something like this in real hardware OR in emulators?