Bare PCB U2F Security KeyI got one of these keys from Amazon. They are used for 2 factor security with websites like Google and Github.
There are many different styles of these keys. Some look like little USB thumb drives. Others like the Yubico or Feitian are plastic encapsulated. This one is a bare PCB with a physical switch.
The U2F Zero is a single purpose keyboard with one key switch. It uses the USB HID protocol and has an RGB LED. It's also one of the cheapest U2F keys available.
The U2F Zero is an opensource project hosted on Github.
I also have Yubikey NEO and Feitian NFC keys. These can be used with an Android phone to do U2F authentication via NFC. There are currently few applications that use them on a phone.
Here is a nice hands on comparison of many of the U2F keys currently available. https://github.com/hillbrad/U2FReviews