How make one login in different sites

Question

Currently I have a lot of sites those requires login. All logins different with each other. Different user name, different password for each person. Now I'm looking for some improvement, like gmail and youtube. You know after logged in www.google.com, other sites like youtube, mail.google.com etc, they login automatically. How they did it?

Answer 1

There are a few different ways of doing this:

Use OpenID. This allows users to use the same username and password as they do on other sites (such as SO). There are multiple providers for this.

Use Facebook's Auth API.

Both of the methods above remove the need to implement your own form of authentication.

However, if you want to maintain your own user list from a central location, see this thread for information on how to implement this type of system.