I can't say, I'm under NDA, but a portion of the VGChartz community would poo-poo it because it's from a rival console maker.

uhh maybe my NES skin on my Wii U or my chainsaw controller for Gamecube

I used to own a Gameboy Player for my Gamecube.

ya gotta love the chainsaw controller

Wave Bird, 2 of them!!!

I can't think of any real accessories right now, but I guess the N64 expansion pack (if that counts) and a fancy flashlight that you could connect to the Gameboy Colour so it would light up the screen. (it's shaped like a snake, really badass)

I don't know if they're cool but:

-Kinect for 360 and Xbox One
-Eyetoy and Live Vision cams for PS3 and 360
-Seaman mic for Dreamcast
-Power Pad for NES (don't have it anymore)
-DDR Dance Mat for 360
-AR Cards for 3DS and Vita
-Circle Pad Pro for 3DS
-Gun cases for Wii
-Keyboard/Mouse for multiple consoles
-Tablet/Phone for Smart Glass/Ouya/PS4

Oh, I also have this:  

Doesn't get much use but it made my Wii look like a PS4/Xbox One before those consoles were even announced!  The right half has a compartment to store controllers, memory cards, etc.

Super Gameboy I think, it gave some of the original games a bit of colour and you can play Gameboy on your TV. I would love to list the Gameboy Player here but I do not own one.

Power Glove for NES, man! It's so bad! :D

Gameboy player(for Gamecube) & Super Gameboy(for SNES) are really best add-on hardware imo.

lol I thought it was an external hard drive looks cool btw.