"Setup multiple codes and track where your new users are coming from with the optional Invitation Tracking Dashboard Widget." see http://wordpress.org/extend/plugins/register-plus/
the plugin doesn't work but has some amazing features I would love to see added. You all should combine some of the cool signup plugins you have all ready developed along with the ones from register plus into one powerful plugin with the option of turning on and off which ones you want. Also, the ability to add custom fields to wp-signup is a must!!!
What do you think?