mu is a collection of utilities for maildirs, in particular for indexing and searching them. mu fills a database with information about your emails, which can then be queried using an easy query language. It can work stand-alone or be integrated with a mail client. It also provides an emacs-based email client ("mu4e") and bindings for Guile/Scheme.
OfflineIMAP is a tool to simplify email reading. It allows you to read the same mailbox from multiple computers and ensures that your changes will be automatically reflected on all computers. You can use various mail clients to read a single box or read mail while offline with full synchronization when you reconnect. You can read IMAP mail with readers that lack IMAP support (same goes for SSL). OfflineIMAP's multithreaded synchronization algorithm performs between 10 and 60 times faster than many mail readers' internal IMAP support, and it can be used even with mail readers that support IMAP already. There are five available user interface modules, including scripted execution, an interactive terminal interface, and GUI interfaces.
TWIG is a Web-based groupware suite written in PHP, compatible with both PHP3 and PHP4. Its features include IMAP and POP3 email, Usenet newsgroups, contact management, scheduling, shared notes and bookmarks, a todo list, and meeting announcements. Any piece of information can be shared with others via group oriented access controls. By using only plain HTML, it is a simple, cross-platform, fast, and browser-independent way to access or share almost any kind of information, without the complexity or costs of other intranet/groupware packages. It is fully internationalized, with 24 translations.
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.
Cone (COnsole Newsreader And Emailer) is a text-based email client. It seamlessly handles multiple POP3 and IMAP accounts and local mail folders. It is also a simple newsreader. It is designed to be foolproof enough to be used by inexperienced users, but also offers advanced features for power users.
Evolution is the GNOME mailer, calendar, contact manager, and communications tool. Evolution represents the next step forward in GNOME applications. The tools which make up Evolution will be tightly integrated with one another and act as a seamless personal information-management tool.
phpGroupWare (formerly known as webdistro) is a multi-user groupware suite written in PHP. Its provides a Web-based calendar, todo-list, addressbook, email, news headlines, and a file manager. The calendar supports repeating events. The email system supports inline graphics and file attachments. The system as a whole supports user preferences, themes, user permissions, multi-language support, an advanced API, and user groups.
NeoMail is a web-based email client written in Perl. It provides access to a machine's local mail spools without requiring a POP3 server to be running, and without giving users shell access. It supports sending and receiving attachments, multiple folders, user themes and preferences, templates for interface generation, quotas, language translations, etc.