TellYourFriends WP is a WordPress plugin by ExtendedProduct that allows your users to tell their friends about your website – With a simple form in the sidebar!
You can also set a limit of the amount of friends each visitor can recommend, just in case you have any worries about the amount of e-mails you’ll send out.
Just enable the plugin, enable the widget in the Appearance –> Widgets menu and then change the settings in Settings –> TellYourFriends WP!
Download: http://wordpress.org/extend/plugins/tellyourfriends-wp/