The Yubikey NEO (and the 4, but the 4 is more closed-source than the NEO) is basically a Javacard smartcard with OpenPGP and PIV (x.509) applets with standard i

The Yubikey NEO (and the 4, but the 4 is more closed-source than the NEO) is basically a Javacard smartcard with OpenPGP and PIV (x.509) applets with standard interfaces. I have my GPG auth/sign/encrypt subkeys on it (master/certification key stays offline), and with gpg-agent it will present your GPG auth key as SSH credentials. There are Yubikey-specific PAM modules you can use as a second factor for logging in locally, and there are probably ways to use standard smartcard authentication for login purposes as well but I don't have experience with that. I mainly use it for remote SSH login purposes.