I have this annoying habit of sending emails that refer to an attachment, only to realize after sending that I never actually attached anything. So I wrote a plug-in for Mail.app that scans my messages when I hit Send. If there's no attachment, and it looks like I'm referring to one in the body, it pops up an alert to confirm sending.
I've also written up a tutorial on how to write a Mail.app plugin, but be warned! It's aimed at the experienced programmer.
[robg adds: I tested the plug-in and it works as described. I didn't bother with the how-to, given my lack of programming skill!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060330105237123