Shoot, did I previously log in to SO with my Google account? With my Facebook account? How can I remember? Or maybe, I created a "myOpenID" account? After trying various combinations, I became frustrated by the error "No OpenID endpoint found."
Finally, I ended up spending twenty minutes digging through my old e-mail to figure out that I did, in fact, create a dedicated OpenID account at myopenid.com.
It turned out, for some ungodly reason, that I have to enter my OpenID as "http://<my username>.myopenid.com/" -- I would not have ever guessed this.
As a developer, I can appreciate the motivation behind OpenID. But the execution is simply frustrating.