Today was the first time I've used GMail's feature to undo the sending of a message.
I used to think that feature was silly, but today I found it handy; I found that I had chosen the wrong From: address from the drop-down, but didn't notice that until I had already sent the message. So I clicked on the link in the success message: "Message send [Undo] [View message]" and got dropped back into the editor, where I could correct the sender and re-send. Phew!
(If you want to try out the feature, you have to enable the Google Labs "Undo Send" feature by Yuzo F. The undo link only appears for a few seconds before the email is sent out for good, but often, those seconds are all you need!)