The Contacts Sidebar extension for the email software Mozilla Thunderbird displays the address book - or books - in the sidebar of Thunderbird beneath the mail folders. A real beauty is that this sidebar can be enabled and disabled with the F4 button which means that you only need to display it when you need the address book. All the possibilities of the address book are given in the sidebar as well, you can edit properties of your contacts or quickly choose a contact that you want to write a mail to. Here are some highlights taken from the description: