October 15th, 2008 | Published in Gmail (Google Mail)
I often send messages where the subject is the entire message (e.g. "Want to grab lunch at 12:30?"), and Gmail would always prompt me to add in body text.
Now, however, you can add "EOM" or "(EOM)" at the end of the subject line (short for End Of Message), and Gmail will silently send the message without the unnecessary prompt.