OpenID - How to find out OpenID URL

Any of us easily owned a dozen accounts on different services on the internet. Be it Google, Yahoo, WordPress.com and others, these are just a few well known service providers. How about others? e.g. Slashdot (http://slashdot.org/), Stack Overflow(http://stackoverflow.com/), Server Fault(http://serverfault.com/) and the list goes on.

Luckily these sites are equip with OpenID. OpenID make our life easy by having 1 single sign on account (registered with the service providers) authenticate against multiple web sites.

e.g. an account with Google (or gmail) can be sign on into Slashdot, Stack Overflow, Server Fault and others.

Often, these website equip with OpenID sign on service have this "OpenID URL", but how do we get this "OpenID URL"? Or, what is this "OpenID URL"?

Here it is, the "OpenID URL" :