The Ventriloquism Charm (ventriliquo) is a spell used to cause the caster's voice to issue from another location. Its first appearance, while non-canonical was in Ch. 11 (Omake Files 1, 2, 3) by Fred Weasley to save Harry from being sorted into Slytherin:
'"SLYTHERIN!" Seeing the look of horror on Harry Potter's face, Fred Weasley thought faster than he ever had in his life. In a single motion he whipped out his wand, whispered "Silencio!" and then "Changemyvoiceio!" and finally "Ventriliquo! " "Just kidding!" said Fred Weasley. "GRYFFINDOR!"'
It was canonically used by Harry in tandem with the Cloak of Invisibility and a voice-altering spell to speak to students anonymously, leading to rumours that Salazar Slytherin's ghost was responsible.