Where can I find a list of OpenID Provider URLs?

I think you're missing one of the points of OpenID - it's a decentralized system that doesn't have a "master list" - anyone can setup their own OpenID server. If software is built such that it follows the OpenID specifications, it won't need to know the URL of the server ahead of time.

Such a list cannot be complete. The whole point of OpenID is that identity management is NOT linked to a single or a small number of providers. So I can give you an other identity provider: openid.basjes.nl. This provider handles the OpenID of exactly 1 user (perhaps 2 if my wife wants one too).

The effect I do see lately is that sites want users to be able to login with a lower threshold. Many of those users already have an account that is (technically) exposed using OpenID, but these users are not aware of this fact nor do they understand what OpenID is.

So what happens is that sites (like SO) add a few 'convenience' buttons that translate (Javascript?) the site specific userid to the corresponding OpenID. But only for the 'top 5' most popular sites. For the rest they have a normal OpenID.

Just my 2ct.

Please don't forget that OpenID Delegation means that I should be able to use any host I own and control to delegate for my OpenID at another provider. That means that beyond just having any number of id providers, from corporate and community driven, to personal or family based, you further have the issue of having delegation allow for anyone's domain or sub-domain constitute an id. E.G. my-popular-blog.com maybe delegated to popularblogger.myopenid.com.

I would propose to use the http://code.google.com/p/openid-selector/ library. It has the most used urls + graphics for each site.

It is what stackoverflow uses.