I have been asked to implement a cross site login solution in Zend Framework.
I have thought about perhaps using OpenID, are there any other solutions to make a login persist across many sites?
In some ways a closed system hosted by us would suit better.
I would go with openid (see http://framework.zend.com/manual/en/zend.openid.provider.html) but you can also make the login action connect to common database...
I think you can't easily make user connected across different domains as long as the cookies are domain specific...
I think the login solution implemented right here at StackOverflow is pretty sweet. It offers a variety of authentication services. Personally, I feel like providing Google and/or Facebook as the auth provider will cover a large percentage of users.
As tomas.fejfar mentioned, it would be tough to use a non-OAuth solution due to browser security.