Here is the script which can be used to create a login with facebook option.
<fb:login-button>Login with Facebook</fb:login-button>
The app id can be obtained from here : http://www.facebook.com/developers/
Here is one interesting thing which i found.. We can literally know what font is being used by doing the following :
First create a bookmark and in the place where you put the url to any site, you need to put this script :
This is a print_screen view of how to add the bookmark..
And save it and click on the bookmark.. Email me for queries regarding this..Its a great tool for everyone thats linked with web design..