Roundcube

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.

http://www.roundcube.net/

Plugins

• http://trac.roundcube.net/wiki/Plugin_Repository
  
• http://code.google.com/p/myroundcube/
  

Login via Basic Auth

Plugin http_authentication

http://trac.roundcube.net/ticket/1485076