I am looking into creating a global ID system for all of our online products including our own company web site (we don't have any products that are public quite yet, but we are fairly close). If we were completely a .Net shop this wouldn't be much of an issue, but we have at least ASP.Net MVC and Ruby on Rails, and who knows what else we'll have in the future.
How would you implement this type of system in a diverse environment? Would you use an internal web service for authentication?