I couldn't find this documented anywhere, but I'm not sure what you would call it, so maybe I was just looking for the wrong thing.
In Mail.app (Leopard, not sure of Tiger), when you forward an HTML-encoded email, you can forward only portions of the message. When you click on sections of the message, a border is drawn around each section of the content (based on the HTML block structure of the message) with close/delete widget in the upper left corner (like those to delete Dashboard widgets). Click the close box, and you'll remove that section of the original email.
This seems like it could be occasionally useful when you want to forward only forward some snippet of an email, without graphics, footers, or possibly ads.
