Quietus
Makes a magically magnified voice return to normal. Used in Harry Potter and the Goblet of Fire by Ludo Bagman.
Reducio
Makes something smaller. Moody uses it in Harry Potter and the Goblet of Fire. Countercharm to Engorgio.
Reducto
The Reductor Curse can blast solid objects out of the way. In Harry Potter and the Order of the Phoenix, Harry remarks that thanks to the D.A. (Dumbledore's Army), Parvati Patil had produced such a good Reductor Curse that she manages to turn a table into dust.
Relashio
Used against Grindylows. Above ground it ejects sparks from the wand; below water it ejects boiling water from the wand. First used by Harry in the second task of Harry Potter and the Goblet of Fire.
Reparo
This spell fixes broken objects. Seen in most books, starting with Harry Potter and the Philosopher's Stone.
Rictusempra
Tickling Charm. Harry uses on Malfoy in Harry Potter and the Chamber of Secrets in the duelling club. In the Chamber of Secrets movie, Harry used this spell to fling Malfoy through the air and in the game Rictusempra is used to stun people or animals.
Riddikulus
Spell used when fighting a Boggart, it forces it to take the appearance of what the spellcaster is thinking as amusing so that the laughter will weaken it. First seen in Harry Potter and the Prisoner of Azkaban.
This is a Latin-based incantation. The word "riddiculus" is a Latin noun (nominative, singular, masculine) and means "exciting laughter" or simply "laughter". Its is pronounced with a hard "c".
Scourgify
Used to clean something. Used by Ginny in Harry Potter and the Order of the Phoenix to clean up Stinksap in the Hogwarts Express.
Serpensortia
Conjures a serpent from the spellcaster's wand. Used in Harry Potter and the Chamber of Secrets by Malfoy.
Silencio
Makes something silent. First used by Hermione in Harry Potter and the Order of the Phoenix.
Sonorus
Magnifies the spellcaster's voice. Ludo Bagman uses this in Harry Potter and the Goblet of Fire to commentate at events without needing a megaphone.
Stupefy
The Stunning Charm. It puts the victim in an unconscious state. Described in the novels as a beam of red light. Countered by Enervate.
Tarantallegra
Makes victim's legs dance uncontrollably (possibly the tarantella dance). Malfoy uses it in the duelling club in Harry Potter and the Chamber of Secrets. A Death Eater also places the spell upon Neville Longbottom in Harry Potter and the Order of the Phoenix.
Waddiwasi
Used by Remus Lupin in Harry Potter and the Prisoner of Azkaban to expel a wad of gum from the key hole Peeves put it in.
Wingardium Leviosa
This spell levitates objects. First seen in Professor Flitwick's class, when the first years practise the spell with feathers. "Wingardium" is a combination of the English word "wing" and the Latin word "arduus" (meaning "steep") and "leviosa", which contains the Latin word "levare" (meaning "ease, lift, pick up").