Yippie Move - The world's best email transfer tool
Mail Backup X - Your one stop mail backup and archiving tool for Mac.
imapsync - Console-based utility for migrating IMAP mailboxes.
Piler - Piler is an open source email archiving solution with all the necessary features for your enterprise
Gmvault - Backup the emails in your Gmail account.
MailSteward - MailSteward : The Ultimate E-mail Manager for Mac OS X