Xuheki is a powerful Webmail application that could compete with desktop-based email clients. It supports multiple users, multiple IMAP servers per user, multiple folders per IMAP server, full MIME support (HTML email, attachments, various encodings), print, drag'n'drop, context menus, easily translatable, etc. Xuheki runs in a browser, but it requires no page reloads. It's free under the GNU GPL.