This may be an obvious one, but I was searching through some folders I had imported into Mail when I moved from PC to Mac, and wanted to search for a person who had sent me email. Now, you can turn on the organise by thread function (View -> Organise By Thread), but that only organises by subject line.
If you want to find all the messages in your Mail app from a specific person across many different subjects, and dont want the pasky trouble of typing a name (which could bring up funny results) or finding an email address from Address Book, here's a shortcut. Simply find any one email from the contact you wish to isolate, then click and hold on the name/address pair in the From or CC header, and literally drag it into the search area, and there you have it, instant search.
[robg adds: The nice part about this trick is that Mail runs the search as soon as you drop the email address. Also, while testing this one, I found an even quicker shortcut -- just drag the message itself from the "one-line" view (top pane) into the search box; Mail automatically puts the email address in the search field.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050828182024581