imapsync - Console-based utility for migrating IMAP mailboxes.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
MailStore - MailStore Home - A 100% free single-private-user desktop solution
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
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.
Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.