The latest Nintendo Switch firmware, version 4.00, offers new features like video recording and USB headphone support, but there's still more hidden deep within. Intrepid folks have found out that you can connect the USB GameCube controller adapter that Nintendo released alongside Super Smash Bros for Wii U. Once a GameCube controller is connected, it'll be seen as a USB gamepad on the Switch.
The Switch support for the GameCube controller extends to all games, you just have to make due without the ZL button. This means you'll be lacking features found in certain games, like Link's shield in The Legend of Zelda: Breath of the Wild. The fact that it works with every game actually makes it better than the original functionality on the Wii U, which only worked with certain titles.
Other USB controllers aren't supported by the newest Switch firmware, so don't try to hook up a DualShock 4 or Xbox One Elite controller to your Switch.
Fans are still waiting for Super Smash Bros to make it to Nintendo Switch in some fashion. Given the re-releases of Mario Kart 8 Deluxe and Pokken Tournament DX, folks expected that Smash wouldn't be that far behind. With GameCube controller support, maybe the dreams of Smash for Switch are a step closer?