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.
Gmvault - Backup the emails in your Gmail account.
audriga Email and Groupware migration - Easily migrate your email, contacts, calendar items and tasks to your new provider or server. No software installation is required.
Mail Archiver X - Mail Archiver X protects one of your Mac-based small business' most important assets.