I'm trying to build a "system" to keep people logged in across multiple sites. It will be similar to google checkout, or microsoft passport, in that a third party is hosting the data, but its different in that the login will need to occur in the middle of a checkout process, and it will also need to keep the login persistent across multiple sites. I read something about using cross domain cookies, but it looks like that would require modifications to the Apache config. The problem is that I will not be able to mess with the "client" sites. Is there any way to do this purely web based (ie. no toolbars in your web browser).