The "Where can I use it" (http://openid.net/where/) does not reveal an answer to the question asked.
I have an openid but I cannot use it anywhere that counts. Clearly this is true because the link above does not show anywhere that I can use my OpenID. What has to happen for Google or Yahoo or Facebook to accept MY openid, not just their own?
AdminChris Messina (Admin, OpenID) commented
Wow, you're totally right. That's a pretty big oversight. Thanks for bringing it up. I believe we had plans to provide a directory, but I don't think we ever made good on them.
For now, there is a directory maintained externally:
It's true that we should have this functionality in the site itself. Thanks for bringing it up.