SirAdmin

SirAdmin is essentially a GUI replacement for cyradm, a command line tool that ships with the Cyrus IMAP server.

SirAdmin allows you to locally or remotely administer your Cyrus IMAP server.
SirAdmin allows you to create, rename, delete, and reconstruct mailboxes, and also allows you to set Access Control Lists for IMAP mailboxes. This is perhaps the most useful function, in that as an admin you can create globally shared mailboxes, or as a user you can choose to share out parts of your IMAP mailbox hierarchy with other users.

It also gives you an easy interface to be able to configure “plus addressing”, where you can send emails topostuser+aSharedMailbox@your.mail.domain and have them be delivered directly to a particular mailbox, rather than to the inbox of a particular user.