How to create a Shared Login Service across Multiple Domains?

Question

I\'m interested in how to implement a shared cross-domain login system as well as best practices and security precautions to take. If you are familiar with 37Signals, you ar

Answer 1

You could try to implement a Global Network Auto-Login solution à la Stack Overflow.
There's an insightful post regarding its brilliant implementation by Kevin Montrose here.

It would require Cookies, HTML5 localStorage, Javascript, Iframe and a lot of security checks but I think it's worth it.