Single Sign On - how to implement?

Question

Say I have a network of websites. When a user logs in to one, I want them to not have to log in again on the other websites. Is there a secured way to do this?

Answer 1

You could do as stackexchange does and implement OpenID.

That'd allow your users to sign in using one of their favorite services, and can be a two-click sign on experience. They wouldn't even need to fill out a form to register for your sites then.

Answer 2

log in with facebook, another option.

Answer 3

Create a domain name that you will use primarily for tracking your network users and use a web beacon image on your login pages to see if a cookie for your tracking domain exists.