imapsync - Console-based utility for migrating IMAP mailboxes.
Roundcube - Web-based IMAP email client
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
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.
HEY - Email at its best, new from Basecamp.