WP Facebook is the ultimate Facebook plugin for your WordPress blog. It integrate Facebook’s Social Plugins into your WordPress blog using WP Facebook’s built-in, customizable widgets.

It allow users to register on your site by having them log in through Facebook! Make it easy for users to connect to your WordPress blog and allow them to use advanced Facebook-exclusive features like attaching comments when using the “Like” button, showing which of your user’s friends are connected to your website, and allowing them to comment on posts on Facebook!

If users are logged in via Facebook, you can choose to replace WordPress comments with Facebook’s Comment System, bringing the discussion to the place where people are actually talking – Facebook! Spark some conversation by taking the convo to your users’ friends and families.

Want to place a Like button directly in an article? Simply use a shortcode to quickly place the button exactly where you want it! All social plugins have shortcode support enabled.

Pricing: FREE
Requirements: WordPress 2.8+
Source: http://upthemes.com/2010/06/wp-facebook-plugin/