[m4dc4pXXX]

https://tozny.com/ sells a service that enables this for any app, not just Google.

I have a Yubikey and I find it too obnoxious for day-to-day use. I'd rather use an authenticator app (such as Google's or LastPass').

For TOTP on non-Google sites, I find LastPass better than Google's Authenticator. First, LastPass locks the app with a PIN. Second, it can work with the browser extension to fill out those 6-digit codes for you.

[oarsinsync]

https://duo.com/ also provides a service (with free and paid tiers) to enable this in a range of applications.