PoCoMy: Postfix + Courier + Mysql :: Webfrontend for administration. Features: Virtual domains, forwarding, aliases, sender aliases, multi user, multi language, privileges system, delegation of rights per domain, ...
Mailcheck is a simple, command line, configurable tool that allows multiple mailboxes to be checked for the existence of new mail messages. It supports both mbox and maildir-style mailboxes. It also supports remote POP3 and IMAP mailboxes.
MadMail is a simple Webmail client which can handle POP3, SMTP, and IMAP servers.
It is written with PHP (Sessions) an can displays HTML messages and MIME attachments.
Features multi-language support
No Database required
LIMeTool (pronounced like the fruit), is intended to be a simple and easy interface for managing accounts on an integrated Sendmail/LDAP/Cyrus IMAP installation. It's written in PHP and is realitively easy to configure and expand.