If you forget your OpenID provider URL, just use your own.
If you have your own domain, from a blog or website, you can use that as your Open ID login. All you have to do is to add two lines of html to your blog or website.
Here is an example for claimID (my open ID provider):
<link rel="openid.server" href="http://openid.claimid.com/server" />
<link rel="openid.delegate" href="http://openid.claimid.com/username" />
Here is some more info from claimID,
but this will work with other providers as well.
Rather than logging in with
http://claimID.com/username, you'd log
in with http://myblog.com. Of course,
claimID will still be providing the
backend (no need for you to install
your own OpenID server!) so you'll be
asked for your password by claimID.
However, when you show up as a
commenter on a Livejournal thread, for
example, your blog or website will be
your OpenID url.