I recently found this neat website and discovered several cool projects that I haven't seen on any other websites. I used this website to create a SNES-to-NES adapter to use my SNES controllers on my AV Famicom. The guides are very well written. If you don't want to DIY, there is also an online shop (second link) for several hacks/mods.

http://www.raphnet.net/electronique/electronique_en.php

http://www.raphnet-tech.com/products.php


 

I have his Gamecube to N64 controller adapter and it is really amazing. I would highly recommend his stuff!