Replex is an administrative tool for managing email on any Unix-like OS. It can handle multiple domains, users, mailing lists, etc. without requiring creation of Unix user accounts for mailboxes or authentication. Replex is written in PHP4 and stores all information in a MySQL database. Replex relies on Postfix for SMTP and Cyrus for IMAP and POP3 accounts.
eXtremail manager is a PHP-based SMTP/POP server manager, specifically for the eXtremail server. It offers the server admin many ways to monitor server activity, and add, edit, or delete users and domains. It also allows the main admin to specify admins per domain, which allows those "mini-admins" to administer their own domains, and none of the other ones.
PHP Mobile Mail is aimed at mobile professionals who want to use their favorite MUAs (noteably pine or mutt) on their laptop, either in UNIX or Windows (under GNU Gygwin). This lets you work offline. It includes a mail_spooler program, which accepts the output of you MUA and spools (on, or offline). When connected, another process parses through the spool and sends your messages. It is also designed to "fake" using your REAL email address with free services like gmx.net, or mail.yahoo.com, when using their POP and SMTP servers, assuming you have a domain and account that you can control to this degree.
eXtended Account Management System (XAMS) allows Administration of Unix, Web, FTP, DNS, and email accounts to manage a provider's typical userbase. Especially focusing on email account management, it enables you to easily create accounts, set aliases, and delegate domain administration to 2nd party resellers or customers. It uses PHP with MySQL or PostgreSQL as a backend, and is therefore able to manage tons of email accounts independently from your Unix/Linux accounts. Exim for SMTP and Courier for POP/IMAP are fully supported, but postfix, Cyrus, and qmail can be used too.
UMPL for XmailServer is a Web frontend for managing Xmailserver and Webmailsolution. Different frontends are presented for root, the postmasters of virtual domains, and end-users. It includes support for managing finger, POP3, SMTP, and Webmail in German, English, Dutch, Italian, and Spanish.