Ruby on Rails Single Sign On with multiple enterprise environments

Let me start off by saying that I'm not totally sure what I'm looking for. I'm hoping I can explain the situation I'm in, and that someone can point me in the right direction. So here's what I'm up against. I want to build a ruby on rails application that will be hosted by oranization A on some domain organizationA.com. Then, multiple other organizations (X,Y,Z) will be accessing this tool from other domains (orgx.com, orgy.com, etc.). I do not know at this moment what form of authentication X,Y and Z will use, most likely it will be LDAP or native Windows, but the important thing is that it could be different from organization to organization. In a situation like this, what is the best way (is there a way?) to allow the users at X,Y and Z to use the application using SSO. Are there any existing tools out there that you would recommend or have worked with? Is there a name for what I'm describing so I can do some better googling? Any help is much appreciated. One Possibility CASino - http://casino.rbcas.com/ Looks like this would be a separate server just for authentication, using the CAS authentication protocol, and could be used for multiple different types of authentication at different organizations. I'll have to check it out.