So we've all heard by now of the new threaded view in Panther's Mail program. But it didn't seem useful to me until I discovered the power of the Command-click.
When viewing one mailbox (say, for instance, your Inbox), you can Command-click on any other mailbox (say, for instance, your Sent folder), and all messages from both mailboxes will be displayed in the main window ... threaded together.
By having both your Inbox and your Sent folders listed together, the threading looks like you'd expect -- your message, the response, your follow-up, the backlash, etc.
An overly simple process to magnify the effectiveness of a new feature.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030926182421980