Roundcube - Web-based IMAP email client
imapsync - Console-based utility for migrating IMAP mailboxes.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
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.