List of compatible USB devices

• Hori LAN Adapter (with RJ45) : http://stores.horiusa.com/lan-adapter-for-nintendo-switch/
• 8Bitdo joypads : http://www.8bitdo.com/#Products
• Hori Pokken Tournament Pro Pad : http://stores.horiusa.com/pokken-tournament-pro-pad-limited-edition-controller-for-nintendo-wii-u/
• Nintendo Switch Pro Controller
• Mayflash's GameCube Adapter : http://www.mayflash.com/products/nintendowiiu/w012.html
• Nintendo Wii U GameCube Adapter (since 4.0.0) : http://nintendoeverything.com/switchs-4-0-0-update-now-allows-for-gamecube-controller-support-with-wii-u-gamecube-adapter/
• Brook GC to SW Controller Adapter : http://www.brookaccessory.com/detail/93354732/
• Brook PS3/PS4 to Wii U Super Converter : http://www.brookaccessory.com/Super%20Converters/list.html
• USB Keyboards
• USB-C OTG
• USB headsets (since 4.0.0) : https://www.reddit.com/r/NintendoSwitch/comments/77dc85/400_update_added_support_for_wireless_usb/

For Ethernet see here.