Roundcube 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.
Installation is easy in Debian but the application is pretty outdated in the squeeze apt repository so you may want to install the latest version manually instead. You can download the source from Sourceforge.
1. Install roundcube
apt-get install roundcube roundcube-mysql
2. Configure apache (pico /etc/apache2/conf.d/roundcube)
Alias /roundcube/program/js/tiny_mce/ /usr/share/tinymce/www/
Alias /roundcube /var/lib/roundcube
3. For some reason the default language is set to ar_SA. Change it to English (pico /var/lib/roundcube/config/main.inc.php)
$rcmail_config['language'] = 'en_US';
4. Restart apache
Now you can access Roundcube on this location: http://yourserver/roundcube