I use Apple's Mail for personal email and Microsoft's Entourage for work email. Sometimes when I'm checking mail on the go I don't have time to read all my emails. So based on the subject and who it's from I may skip around. When things settle down and I have more time I want to go back and read all the ones I haven't read yet. Sure I can scroll through the list looking for the ones that are still bold, but I want them all in one easy window.

In Mail I created a "Smart Mailbox" that just shows me unread mail

In Entourage I hit Command-Shift-o and it will change the view to just the unread messages.