imapsync - Console-based utility for migrating IMAP mailboxes.
Roundcube - Web-based IMAP email client
IMAP Upload - IMAP Upload is a tool for uploading a local mbox file to IMAP4 server.
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.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.