I love Mail.app on OS X. I use it religiously (now that they added ‘Smart Folders’) – but if you use Apple Mail.app you have the check out MailTags by indev. Its a great little bundle that helps you really manage all of the email madness – here is a snippet from their site …
MailTags is a plug-in for Apple’s Mail.app 2.0 (Tiger) that enables you to go way beyond folder filing for organizing mail by enabling metadata tagging.
I just tried this out and have to give it my kudos too. It’s great.