Google authenticator for the desktop
There are Java apps, both GUI and CLI. Both were listed on the Google Authenticator page at Wikipedia. Both include install instructions on their respective download pages.
I like the idea but I have not tested either of these nor would I assume they are secure or even work correctly. This is a summary of my search results, not a recommendation.
Simply install the small command line utility oathtool.
sudo apt-get install oathtool
Then run such a command to get a one time password:
oathtool --totp -b YOURSECRET
You can use some Extension for your web browser, they work pretty good.
Here you go, here's an example:
For Chrome/Chromium/Yandex (and other Chrome based browsers) you can use Authenticator. Just click on "Add to Chrome" button from this link.
Or you can add it through: Customise and control menu (three dashes/dots upper right corner of the browser) > Settings > Extensions > Get more extensions (in the bottom of the list) > Search the store for: Authenticator.
You need to add a Passphrase. This step will allow you to sync multiple instances - transfer of the authentication data between computers which you use with your Google account.
To add an authentication key: First click here to open a demo barcode in your browser. Then follow next steps for: Scan QR code or Manual entry. If the barcode is not generated by some Google application, for most cases you must use only Manual entry. If the authentication code is time based you must use Sync Clock with Google function.
To use the application, just click on the Authenticator icon. Wait for the beginning of a new cycle. Click on the new generated key. It will be copied to the clipboard.
Use the key! Usually you have about a minute and a half to use it.
I'm not so familiar with other browsers, but for example, within Firefox in a similar way you can use Open Two-Factor Authenticator.
In addition, the next answers describe how to implement Two Factor Authentication (using Google Authenticator) within Apache2 and OpenSSH:
How to secure phpmyadmin
Secure an Ubuntu OpenSSH server from Brute force attacks but without a firewall or SSH key pair?