Apple Mail has this and it's very useful. Quickly view All of today's mail, regardless of which folder it may be in. Filtering mail to folders is great, sometimes I need to quickly locate an email message from today. As a new feature, it could be designed to display messages from 'today' or a rolling '24 hours', for example. I personally like the 24 hour idea better.

Not having this feature is what prevents me from building my desired filter system. It's too inefficient for me to continuously check mutliple folders to identify messages that came in today.