These webmail clients can operate under different web platforms(PHP, ASP.NET, Java). Here below is the list of 7 webmail clients.
1. Roundcube is a browser based multilingual IMAP client. Its features are MIME support, address book, folder manipulation, message searching, spell checking, contacts searching, threaded message listing, IDNA, shared folders, etc.
2. Zimbra is a browser based AJAX client which gives you message conversation view and visual search builder.
3. Atmail offers a webmail client, email server platform and mailserver appliance for Linux, Unix and Windows.
4. SquirrelMail is an Open Source software written in PHP4 which provides both a web-based email application and an IMAP proxy server.
5. Hastymail is a full featured, fast, secure, compliant IMAP or SMTP web mail client written in PHP. Its features include compatible with PDAs, phones, text browsers, Multilevel theme system, Support for Postgresql and MySql, Flexible plugins, tunable smart cache and many more.
6. Xuheki Webmail is a fast open source Browser based Ajaxed IMAP client which aims to be your preferred email client.
7. AfterLogic is fast and easy-to-use webmail script for POP3/IMAP mail server or cPanel.It offers mail server, ajax webmail with calendar, mail components, personal user settings, rich text editor, spell checker, mail filters and search, development and installation services.
Besides, these 7 webmail clients there are many others options for you. I have posted those which i like, you may have your own choice.