Gmvault - Backup the emails in your Gmail account.
imapsync - Console-based utility for migrating IMAP mailboxes.
Mail Backup X - Your one stop mail backup and archiving tool for Mac.
Cryoserver - Cryoserver is an all-in-one email archiving solution that empowers you to preserve your email in a tamper-evident archive, making you transform your data into a useful archive for everyday use.
Mail Archiver X - Mail Archiver X protects one of your Mac-based small business' most important assets.
MailSteward - MailSteward : The Ultimate E-mail Manager for Mac OS X