Implement Open Id login

I am curious to know how can i add the Open ID login to my website, like the one this开发者_JAVA技巧 site (stackoverflow.com) is using.

My intention is to enable users to login into my site, without registration. So if they just have an account at popular sites like google, yahoo or facebook, they must be able to login to my website.

Can anyone suggest me a good and free solution ?

In stackoverflow here are answer

How to use open id as login system

How do I implement OpenID in my web application?