A lot of us might prefer to use official controllers for retro games, but not everyone feels like having a ton of different controllers sitting around. Ben Heck decided to make a universal controller.
There's a lot going on here, including a 3D printed case, a bunch of different parts on the inside, and a Wi-Fi module. He also makes custom receivers that plug into a Nintendo, Super Nintendo, Genesis and an Atari. This all works by essentially emulating each controller type inside the DIY controller itself.
As is often the case of Heck's builds, it's a bit over-engineered, but it's still a solid blueprint for anyone looking to build their own controller.
Ben Heck's Multi-System Retro Controller [YouTube]