Similar to Mail Merge is the desktop mail list manager, like one of these, or macjordomo for the macintosh.
The biggest difference between desktop mailer software and mailmerge is automated list management. Working in conjunction with a special subscription mailbox, such as mailinglist@yoursite.com, subscribe and unsubscribe requests are made by sending an email to that address. You can also import a list of addresses into the list manager. When you run the program on your desktop it checks the mailbox for new subscriptions. When your computer is not connected to the Internet, all the subscription requests will be put on hold until you connect again.
There will also be a second mailbox for the list manager, for email broadcasts. To broadcast an email to your list you send a message to the second mailbox and that message gets reflected out to all of your subscribers.