The number of providers listed on Spread OpenID is growing even more. We have added StartCom SSL to the provider listing today.
StartCom SSL allows login by client certificates and smart cards. It is a welcome trend that OpenID providers are adding more secure layers to the usual username/password login method to their services. This will hopefully further the acceptance and use of OpenID even more.

2 Comments
It looks like it might be down now, but http://prooveme.com/ was an interesting implementation of SSL certificate based OpenID.
Of course, the key reason to use SSL (with or without client certificates) is that it allows you to validate the OpenID provider and ensure that man-in-the-middle attacks are more difficult to implement.
But I expect y’all knew that.
Hm, the website of StartCom SSL is available. Don’t know about the provider, though.