Developer website: James Eagan
ASP (Attachment Scanner Plugin for Mail.app)... 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, it's invaluable.