I've quickly checked out ZohoMail and it looks nice. IMAP isn't supported yet though, so that makes it less than ideal. POP3 does work, and you can easily import other e-mail accounts (it works like an online e-mail client). So suppose you don't like your current webmail interface (e.g. Outlook Webaccess doesn't work with all it's features in FF or Opera), you can simply use ZohoMail as a client. Perhaps GMail can do this is as well, I don't know, but it's a nice touch. The interface is pretty okay to, allowing you to easily label your e-mail.
By next march IMAP should be supported and then I might consider using it. For now, it's GMX for me.