We are making a website right now and we would like users to be able to login using their OpenID credentials. After an hour or so of googling I gave up trying to find a list of the OpenID URL formatting for the various OpenID providers (http://username.myopenid.com or http://flickr.com/username for example) and instead went through the MyOpenID site directory to compile my own list. Here it is in all of its glory:

Edit 2-4-2010: Today I went through and attempted to verify that all of these openid providers were good.  The third column in the table represents my findings.

Google:	https://www.google.com/accounts/o8/id *	good
Yahoo:	http://yahoo.com/ *	good
MyOpenId:	http://username.myopenid.com	good
LiveJournal:	http://username.livejournal.com	good
AOL:	http://openid.aol.com/username	good
WordPress:	http://username.wordpress.com	good
Blogspot:	http://username.blogspot.com	must use blog url, blogspot = blogger
Verisign:	http://username.pip.verisignlabs.com	good
ClaimID:	http://openid.claimid.com/username	Signs in but nothing happens, appears to be broken on claimid end
clickpass:	http://clickpass.com/public/username	I was only able to signup with IE and then there was a weird login procedure but it ultimately worked
Google Profile:	http://google.com/profiles/username	good
Blogger:	http://username.blogspot.com/	good
Flickr:	http://flickr.com/username	Couldn't get flickr to work
identity.net:	http://username.identity.net/	Never got sign up confirmation e-mail to test
Bloglines:	http://username.bloglines.com/	Didn't work
Technorati:	http://technorati.com/people/technorati/username	Didn't work, reports of brokenness as of November of last year
Vidoop:	http://username.myvidoop.com/	good
Vox:	http://username.vox.com/	good
MySpace:	http://myspace.com/username	Must make and use account url
Musicpictures:	http://ww4.musicpictures.com/openid/username	Works fine but there is 0 openid advertising on the site
Elgg:	http://explode.elgg.org/username	Tried registering two different places on elgg.org and neither worked. explode.elgg.org no longer exists so I think openid providing went with it. However, there is still an explode.elgg.org option in their openid login so I may be wrong.
MyID:	http://username.myid.net/	good - easiest sign up of any site I visited
IdProxy:	http://username.idproxy.net/	good
Sxipper:	http://username.sxipper.com/	Demo works with trainer id but it didn't work on the actual openid logins I tried.
Signon:	http://username.signon.com/	good
TypekeyTypepad:	http://profile.typekey.com/usernameblog url (form of blogname.typepad.com)	Typekey is now part of typepad so I couldn't verify that typekey still works but typepad definitely does.
Smugmug:	http://username.smugmug.com/	Couldn't get to work with trial account, possibly works after paid?
StartSSL:	https://username.startssl.com/	This site is weird, I kept getting ssl errors when I went to sign up. I was told via e-mail however that startssl is an openid provider and their url is in the format listed here.
Beemba:	http://username.beemba.com/	beemba.com forwards to cliqset which does allow openid logins but as far as I can tell does not provide logins.
Idtail:	http://username.idtail.com	Oriental site of some sort, couldn't get signed up but they do have openid login options

*These sites do not require your username in the login string.

The login procedures at the different sites varied but my rough estimate says about 50% of the sites require you to know exactly what your personal URL is for the provider you are trying to login through (http://openid.aol.com/joojoo2988 to login with aol). Another 45% used some form of the RPX plugin and the final 5% rolled their own system that assisted with a small set of providers and allowed you to enter the URL for the rest (see Stack Overflow). This list is by no means exhaustive and if you know of another url you would like me to add to the list leave a comment or send an email.