Salvio Hexia
Incantation: Salvio Hexia
Pronunciation: sal-vee-oh hek-see-ə
Description: Provides some form of protection against hexes.
Seen: Harry and Hermione cast this spell to strengthen their campsite's defences against intruders in Deathly Hallows.

Scouring Charm
Incantation: Scourigify
Pronunciation: skur-ji-fy
Description: Used to clean something.
Seen: First used by Tonks to clean Hedwig's cage in OotP. Later, Ginny performs the spell to clean up Stinksap in the Hogwarts Express. While looking at Snape's memories, Harry sees James use the spell on Snape's mouth.

Sectumsempra
Incantation: Sectumsempra
Pronunciation: sek-təm-sem-prə
Description: Violently wounds the target; described as being as though the subject had been "slashed by a sword". Created by Severus Snape.
Seen: First seen in OotP when Snape uses it in his memory against James, but misses and only lightly cuts his cheek. Used successfully by Harry in HBP against Draco, and then later against the Inferi in Voldemort's Horcrux chamber, and Snape during his flight from Hogwarts. In the opening chapters of Deathly Hallows, Snape accidentally casts this curse against George Weasley in the Order's flight from Privet Drive, though George was not his intended target. It is known as a speciality of Snape's.
Though Snape was able to mend the wounds inflicted on Draco by this curse with ease, with "an incantation that sounded almost like song", Mrs Weasley was unable to heal her son George when his ear was severed by the curse. Therefore it is presumable that he himself had invented the healing spell for it as well. It was discovered in an old copy of Advanced Potion Making by Harry; Sectumsempra was invented by Snape with the words "For enemies" written next to it.

Serpensortia
Incantation: Serpensortia
Pronunciation: sur-pən-sor-tee-ə
Description: Conjures a serpent from the spell caster's wand.
Seen: Used by Draco whilst duelling Harry in their second year and Voldemort in the duel against Dumbledore in OotP.

Silencing Charm
Incantation: Silencio
Pronunciation: si-len-see-oh
Description: Silences something/someone immediately.
Seen: First used by Hermione in OotP to silence a frog and a raven in Charms class, then later to silence a Death Eater who was trying to use a spell against Harry Potter. It was also used by Voldemort in Deathly Hallows during the Battle of Hogwarts.

Slug-Vomiting Charm
Incantation: Eat slugs
Description: A jet of green light strikes the victim, who then vomits slugs for an undefined period of time (greater than five hours). The sizes of the vomited slugs decrease with time.
Seen: In Chamber of Secrets, Ron attempts to use it on Draco by saying "Eat Slugs"; the spell backfired and hit him instead. Mentioned in OotP before Gryffindor's first Quidditch Match against Slytherin when Draco taunts Ron, "Harry was reminded forcibly of the time that Ron had accidentally put a Slug-Vomiting Charm on himself".

Sonorus
Incantation: Sonorus
Pronunciation: son-nohr-əs
Description: Magnifies the spell caster's voice, functioning as a magical megaphone
Seen: By Ludo Bagman and Cornelius Fudge in Goblet of Fire to commentate at the Quidditch World Cup and during the Triwizard Tournament.
The counter-spell is Quietus.

Scarpin's Revelaspell
Incantation: Specialis Revelio
Pronunciation: spes-ee-al-is re-vel-ee-oh
Description: Causes an object to show its hidden secrets or magical properties.
Seen: Used by Hermione to find out more of Harry's Advanced Potion-Making book in HBP. Used by Ernie Macmillan to find out the ingredients of a potion.
Used by Newt Scamander in Fantastic Beasts and Where to Find Them (as only revelio) to reveal the true identity of Percival Graves.

Stealth Sensoring Spell
Incantation: Unknown
Description: Detects those under magical disguise.
Seen: In OotP, Umbridge casts this around her office. Also used at the entrance to the Ministry of Magic.

Stinging Jinx
Incantation: Unknown
Description: Produces a stinging sensation in the victim, resulting in angry red welts and occasionally the severe inflammation of the affected area.
Seen: Hermione casts the Stinging Hex on Harry in Deathly Hallows to purposefully distort Harry's appearance.

Stunning Spell
Incantation: Stupefy
Pronunciation: stew-pi-fy
Description: Puts the victim in an unconscious state. Manifests as a jet of red light.
Seen: First seen in GoF, used by Ministry officials at the Quidditch World Cup and later against Barty Crouch Jr. Also seen used by a number of Ministry officials against McGonagall in OotP. It is also taught by Harry in his DA meetings and used extensively during the Battle of the Department of Mysteries against the Death Eaters.
Is seen by some, including Harry himself, as the basic spell for fighting. Death Eaters, Ministry Officials, Order members and students all seem to refer to this spell as their preferred attack.
Notes: Hagrid was able to withstand multiple direct Stunners due to being half-giant, and GoF shows six to seven wizards working in unison to Stun a single dragon.

Supersensory Charm
Incantation: Unknown
Description: Able to possess superior senses than before.
Seen: Mentioned by Ron outside of the Hogwarts Express during the epilogue of Deathly Hallows as a potential substitute for using mirrors while driving a car.

Switching Spell
Description: Causes two objects to be switched for one another.
Seen: Harry contemplates using this spell against his dragon in the first task of the Triwizard Tournament. Neville also uses this in Transfiguration class in GoF, and accidentally transplants his ears onto a cactus.