OpenGroupware.org is a set of applications for contact, appointment, project, and content management. It is comparable to Exchange and SharePoint. It is accessible using Web interfaces or using local clients via WebDAV, GroupDAV, and CalDAV. Custom applications can be developed using the XML-RPC API. OpenGroupware.org runs on almost every GNU/Linux system.
Domain Technologie Control (DTC) is a Web-based control panel for hosting that can delegate the task of creating subdomains, email, and FTP accounts to users for the domain names they own. It has support for many programs, including bind 8 and 9, MySQL, Apache, PHP 4, qmail, Postfix, Courier, Dovecot, ProFTPD, Webalizer, and mod-log-sql. It can also generate backup scripts, calculation scripts, and config files using a single system UID/GID, and monitor all traffic accounting per user and per service. It is fully skinnable and translated into several languages.
Dovecot is an IMAP server whose major goals are security and extreme reliability. It uses index files to optimally store the mailbox state, which makes it very fast even with huge mailboxes. Indexes won't prevent external mailbox updates, so Dovecot is still fully compatible with standard Maildir and mbox formats. There's also a fully featured POP3 server included.
Ristretto is the mail API that is used by Columba, a Java mail client (http://columba.sf.net). It is available as a stand-alone API, and it provides a well-tested, slim, and fast Java implementation of all relevant mail protocols and features. It is licensed under a MPL/LGPL/GPL tri-license.
Buni Meldware Communication Suite is an enterprise groupware product providing an email server, calendar server, and a Web mail client. The Web mail portion is written using Adobe's Flash and provides a rich cross-browser experience. Protocol support includes IMAP, POP, SMTP, iCAL, WCAP, and more. It uniquely supports full database email storage as well as file system storage of emails on major database platforms, and is easily installed via a Web Start graphical installation process.