MailStore - MailStore Home - A 100% free single-private-user desktop solution
MailSteward - MailSteward : The Ultimate E-mail Manager for Mac OS X
Gmvault - Backup the emails in your Gmail account.
Mail Backup X - Your one stop mail backup and archiving tool for Mac.
Got Your Back - Got Your Back (GYB) is a command line tool that backs up and restores your Gmail account.
Piler - Piler is an open source email archiving solution with all the necessary features for your enterprise