The Big Brother DataBase is an Emacs Lisp program which
keeps addresses (can handle several locations, several phone/fax/mobile numbers, several email addresses).
The nice thing is that it makes the email addresses available for completion in Emacs mail readers. It also adds any previously unknown email addresses to your address book when it spots them in your (emacs) mail reader. For a truly powerful mail system, use Emacs Gnus with BBDB, Supercite and Mailcrypt.