Merlin Mann over at productivity blog 43Folders is taking a look at the Smart Folders feature of Apple's Mail today. His favorites include a folder to hold "flagged" messages, a recently sent folder (within the last two days) and recently viewed (same criteria).
The one I thought was clever was a collaborative effort between Address Book and Mail. In Address Book, he created a group based on geographic location (particularly, San Francisco). Back in Mail, he created a smart folder to group messages received from contacts in the "San Francisco" group. That's a pretty good idea.
I'm sure you see the question coming: What are your favorite smart folders setups? I actually don't use any, as I want an empty mailbox. Any message that I think requires me to somehow do something I forward to my Backpack "Inbox" page, where it waits for me to decide what it is and act accordingly. But that's me.