WordPress.org

This plugin displays a greeting message to visitors that come from different urls (known as referrers). For example, you may want to welcome Diggers with a message that reminds them to digg your story, or you may want to ask Del.icio.us users to bookmark your post, and so on.

Not only about a cool greeting box, it greatly helps to more efficiently interacts with your readers and build a better relationship between you (your website) and them.