I really like using roundcube webmail.
I have always thought that a brilliant feature that would make any email system stand apart would be a notes feature. Imagine if you had a notes window for each email. When you click on an email you can see any notes you have left previously. So if I get an email from my boss asking me to do 3 things I can keep a record of what I have done to act on the email.
I would love this feature.
Kind regards, Chris Hodson
www.GOZONE.info
List info: http://lists.roundcube.net/dev/