Single Login across multiple sites

I am anticipating being asked to createa single login system for my coldfusion site and other sites(not coldfusion) on different domains.  The other sites will not be tasked with this, but hopefully there will be some cooperation with the other vendors(there has to be).  How would I even begin to research this?  I was thinking about somehow passing the login info from the coldfusion site to the new site and logging in in the background.