imapsync - Console-based utility for migrating IMAP mailboxes.
OfflineIMAP - OfflineImap synchronizes email between an IMAP server and a MailDir or between two IMAP servers.
Gmvault - Backup the emails in your Gmail account.
Fetchmail - The Fetchmail Project
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.
IMAPSize - IMAPSize