Hastymail is a full featured IMAP/SMTP lightweight webmail client written in PHP. It's features includes: the ability to use a database for storing user settings, contacts, and plugin data, flexible and powerful plugin system with documentation and examples, small page sizes to minimize load time and bandwidth use, multiple interface languages and enhanced message character set handling. Also, the client has advanced search capabilities.
Horde Groupware Webmail Edition is a free, enterprise ready, browser based communication suite. Users can read, send and organize email messages and manage and share calendars, contacts, tasks and notes with the standards compliant components from the Horde Project.
Mailr is an open source webmail application written in Ruby which uses Ruby On Rails web application framework. The application is tested with Courier-IMAP but it should be possible to use it with any other IMAP server.
Postaci is a PHP based POP3/IMAP e-mail client. It is very simple and easy to use. It supports SMTP authentication and can handle both protocols and the default protocol can be changed from a single configuration file. Postaci is platform and database independent application which works on any operating system which supports PHP. Also, it supports multiple languages: Turkish, English, German, French, Portuguese, Spanish, Polish, Norwegian, Dutch, Italian etc.
RoundCube Webmail is a browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an e-mail client, including MIME support, address book, folder manipulation, message searching and spell checking. RoundCube Webmail is written in PHP and requires a MySQL or Postgres database. The user interface is fully skinnable using XHTML and CSS 2.
SquirrelMail is a standards-based webmail package written in PHP. It includes built-in pure PHP support for the IMAP and SMTP protocols, and all pages render in pure HTML 4.0 (with no JavaScript required) for maximum compatibility across browsers. It has very few requirements and is very easy to configure and install. SquirrelMail has all the functionality you would want from an email client, including strong MIME support, address books, and folder manipulation and it is stable enough to use in a production system.
Xuheki started as a personal project because author needed a webmail system he's own use. Here is a brief list of features that Xuheki already has: the interface is similar to a desktop-based email client and it's built used AJAX with no page reloads, supports multiple IMAP servers and multiple sender identities; the frontend architecture allows it to stay connected to the IMAP server; support for multiple folders with drag'n'drop feature; the interface is designed to allow doing multiple things at once, It has a basic address book and autocompletion of addresses in composer and others.
yawebmail is a webmail-client written in Java as a webapp. Having it deployed, it provides access to POP3 or IMAP Mailbox, even if client computes is behind a firewall. While other webmail-clients may have a lot of bells and whistles, yawebmail concentrates on the support of as many standards as possible.
Zimbra Collaboration Suite features a state-of-the-art Ajax web client- it's the ideal productivity tool for today's enterprise, service provider, academic, and government environments. By outpacing traditional desktop application performance and working in the web browser, Zimbra gives a better messaging and collaboration experience plus zero desktop lock in.