With this add-on your users can link to other accounts(e.g. couples using different accounts on the same forum, admins testing the usergroup permissions with real test accounts, developers, or for authors using an universal news account for the site news....)
You can also allow your users, to see who linked to there account.
In the Style Properties, you can define if it should be shown inside the Account Popup or as own tab
Since 1.4 it's also possible, to don't need the user password anymore.