Important: PowerPC users see note at bottom.
Do you ever forget to include an attachment on your emails? It happens to me all the time. By the time I get done writing my email, I forget that I needed to attach something to it before hitting Send. This Mail.app plugin helps avoid that problem. When you send a message, it checks if there's an attachment. If there isn't, and it looks like you've referred to an attachment in the body of your mail (by using a word like attach, attaching, attachment, etc), it gives you a warning. Most of the time, you should never notice that this plugin is running, but in those cases when you do need it....
Requires Tiger (10.4) or Leopard (10.5) on Intel and Mail.app. There is an older version for Tiger on PowerPC