Another fine piece of magic for everyday living from the Harry Potter Universe. A Howler seems like an ordinary letter, except it's sealed in a bright red enveolpe. Upon opening, the message is shouted in the sender's voice (magnified about a hundred times) to all within a broad earshot, then the letter ignites and burns to ash. Also, if the Howler is not opened after a brief period of time, it explodes violently.
Great for sending embarassing harangues, as illustrated with the Howler Ron Weasley got from his mother in Harry Potter and the Chamber of Secrets for stealing and crashing Mr. Weasley's flying car.