WP Greet Box lets you show a different greeting message to your new visitors depending on their referrer url. For example, when a Digg user clicks through from Digg, they will see a message reminding them to digg your post if they like it.

You can also set a default greeting message for new visitors suggesting them to subscribe to your RSS feed. Having these targeted suggestions will help your blog increase exposure, loyal readership, and reader interaction. Best of all, this plugin is compatible with WPMU and various WordPress cache plugins.

Pricing: Free
Requirements: WordPress 2.6+
Source: http://wordpress.org/extend/plugins/wp-greet-box/