MailStore - MailStore Home - A 100% free single-private-user desktop solution
imapsync - Console-based utility for migrating IMAP mailboxes.
Mail Backup X - Your one stop mail backup and archiving tool for Mac.
Piler - Piler is an open source email archiving solution with all the necessary features for your enterprise
iSync - iSync is a software application published by Apple Inc.
Gmvault - Backup the emails in your Gmail account.