You might have a Google account, and encountered various websites and services who allow you to log in with for example a Facebook account, a Google account or an OpenID account.
As usual, Facebook wants (or rather forces) you to use their services exclusively, so if a Facebook login is the only possibility, you're screwed.
Google however tends to use open systems, and they do this for logins, too. Even if a website requires you to log in using your Google account, it is using Googles OpenID services. Unfortunately you're still screwed: if the site supports the Google account excuisively, it will have the Google OpenID URI hardcoded.
Now for the good news: if a site offers OpenID login directly, you cannot only use any OpenID provider you'd like, you can also use your Google account. Unfortunately, it is (imho) less than obvious - Google doesn't provide a short, practical URI for this purpose. The Google OpenID URI is https://www.google.com/accounts/o8/id.
Not terrible, yet inconvenient, because you'll have to type it in if there is no huge "Google Login" button.
To make it easier, I first tried various link-shorteners, but this ...