Roundcube - Free webmail for the masses. 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.
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 for maximum compatibility across browsers. SquirrelMail has all the functionality like email client, including strong MIME support, address books, and folder manipulation.
WEBinsta maillist manager is built to provide a centralized mailing list / newsletter system for small to medium website . Its simple yet quite powerfull . It support the standard subscribe and un-subscribe , remote verification , unsubscribe links , attachments and host of other such features . The best of all the total integration is less than 3 lines of code .