RoundCube is a free and open source IMAP webmail client with a very intuitive user interface. RoundCube has all the features you would want in an email client – WYSIWYG email composition, address book, support for MIME/HTML emails, multiple server identities, spell checking, drag & drop message management and more.
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Roundcube is a popular web-based mail system that you can deploy on top of your existing infrastructure (i.e. hooking up to your existing IMAP server). It is very popular with universities and ISPs. While not as polished / full featured as commercial services like GMail, it provides much greater flexibility and control over the interface and the data.