There's a little easter egg I found inside Mail. Control-click on the Mail.app file and select Show Package Contents, then go to the Contents -> Resources folder. Find the senders.tiff file and double click on it to open it in Preview.
[Editor's note: The images you will see are those of the Mail team, and they appear automatically if you ever receive an email from one of them; Cricket (the second picture, obviously) posts to a mailing list fairly regularly, and it was quite odd to see that image the first few times it came up, knowing I'd never had a personal email from Cricket!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030110061905539