Gmail – If the Gmail address is abcdefgh@gmail.com, then what would be the OpenID

It seems like this was enabled last year. Here is the blog-post:

http://googlecode.blogspot.com/2009/07/google-apps-openid-identity-hub-for.html

Note: The Federated Login Service is disabled by default for Google Apps Premier and Education Editions. The domain admin can enable it from the Control Panel at http://www.google.com/a/cpanel/<your-domain>/SetupIdp.

The OpenID URL for Google Apps is something like:

https://www.google.com/a/<your-domain>/o8/ud?be=o8

To make changing OpenID providers easy: set up some delegate on a domain you own. You then don't have to change all your accounts, but only need to change your own HTML.

On the Stack Overflow blog, Using Your Own URL as Your OpenID describes this, but does not include the 2.0 syntax. Like for myopenid.com, for both version 1 and 2:

<link rel="openid.server openid2.provider"
  href="http://www.myopenid.com/server" />  
<link rel="openid.delegate openid2.local_id" 
  href="http://youraccount.myopenid.com/" />  
<meta http-equiv="X-XRDS-Location"  
  content="http://www.myopenid.com/xrds?username=youraccount.myopenid.com" />

(Alternatively: take full control yourself, and become your own OpenID provider.)