YubiKey

YubiKey is a hardware authentication device manufactured by Yubico that supports one-time passwords, public key encryption and authentication, and the Universal 2nd Factor (U2F) protocol developed by the FIDO Alliance (FIDO U2F). It allows users to securely log into their accounts by emitting one-time passwords or using a FIDO-based public/private key pair generated by the device. YubiKey also allows for storing static passwords for use at sites that do not support one-time passwords.

There are several services that integrate with YubiKey. I use YubiKeys to secure log-on to my laptop, protect my password manager, and safeguard some on-line accounts. By using YubiKey as part of my security, an adversary now not only would need to crack my passwords, but would also need to gain physical access to my YubiKey in order to access my computer or on-line accounts.

Google's Advanced Protection Program uses YubiKey to provide "Google’s strongest security for those who need it most." . A list of other services where you can use your YubiKey can be seen here: Services that Integrate with the YubiKey.

You can purchase YubiKey for several sources, including Amazon.

YubiKey 4 

YubiKey NEO 

YubiKey 4 Nano 

FIDO U2F Security Key