It used to be http://yahoo.com, but it is failing for me with DiscoveryFailure in python OpenID library (since today, I wasn't testing this earlier). Also this fails if you try to use the SO login with a yahoo button, so I am thinking it probably changed recently.
I noticed it was down a few hours ago. It isn't working for any RP I've tried... except for zoho.com
I viewed their source and saw they are using https://me.yahoo.com, which works.
Yahoo either made a mistake in a recent site change, or forgot to tell anyone (including themselves). http://openid.yahoo.com/ still says to use "yahoo.com" as your openid.