Thunderbird is a total redesign of the Mozilla mail component to produce a cross-platform, stand-alone mail application using the XUL user interface language. It has many new features, among them the ability to customize your toolbars the way you want them. a new look and feel with a large number of downloadable themes which alter the appearance of the client, and the ability to add UI extensions.
SOGo is a very fast and scalable modern collaboration suite (or groupware). It offers calendaring, address book management, and a full-featured Webmail client along with resource sharing and permission handling. It also makes use of documented standards (IMAP, CalDAV, CardDAV, etc.) and thereby provides native connectivity (without plugins) to many clients such as Microsoft Outlook, Apple iCal, the iPhone, Mozilla Lightning, and a plethora of mobile devices via SyncML. It can reuse any existing email and database infrastructure, avoiding long hours of site restructuring.
adx is a minimalistic but massively Web-enabled address book that runs completely in your Web browser. It's OS-independent and supports comprehensive details for each contact entry, including user identifiers and profiles for Skype, Twitter, Flickr, Delicious, Facebook, LinkedIn, studiVZ, instant messengers, etc. It supports the hCard and XFN microformats.
Address Book Server allows you to share your contacts and calendar events with others connected to your network or over the Internet. Each participant synchronizes their contacts with the server hosted on your own system. Address Book Server supports the full Address Book and iCal schema. The server also provides a friendly Web interface and an iPhone client which let you access your contact details remotely.
Csv2vcard is a program for converting CSV (comma separated values, as exported by software such as MS Outlook) files containing contact information to the industry standard vcard format, which can be imported by most mailers and PIM software (optimized for Evolution). It's based on a Perl script by Michael MacDonald of Ximian. Csv2vcard can be used to convert any CSV format, not just the English Outlook output.
mod_psldap is an Apache module that performs authentication and authorization against an LDAP server with LDAP based session management. It also provides Web 2.0 based capabilities to add, edit, move, and create new records in the LDAP store, leveraging XSL stylesheets to offload heavy processing to the clients and reduce bandwidth consumption by up to 95% or more.
XFMail is an X11 application for receiving electronic mail. It implements most of its email functionality internally so that it does not depend on a lot of extra packages. It supports POP3, MBOX, IMAP4, sendmail, and SMTP. It has built-in support for interfacing with PGP, GPG, ispell/aspell, faces, LDAP, and esound. It runs on most Unix variants.