Google has introduced support for OpenID. If you have a Google account (for example if you are using Gmail or Blogger) you can sign in to other sites that have support for OpenID. Use this URL as OpenID-URL to do so: https://www.google.com/accounts/o8/id. (There is no point using the URL as a standalone in a web browser).
Google has previously had criticism that it was not 100% OpenID compatible, making it impossible to use your Google account on some sites. Some say that Google have fixed these problems now (http://google-code-updates.blogspot.com/2008/10/moving-another-step-closer-to-single.html). I'm not sure until I've developed my own site using OpenID (When I do, I will get back on that issue).
Just for fun I used my Google openID account to register a user on the fun site http://stackoverflow.com/ (it is like, www.experts-exchange.com, focused on questions and answers from users in different computer related topics, but free). It worked very well to login to stackoverflow with my Google-OpenID-account.