Accio: summoning charm

Avada Kedavra: unforgivable curse, killing curse

Alohomora: makes locks open

Aparecium: makes invisible ink visible

Avis: lets off "a blast like a gun" from a wand and "small, twittering birds" fly out the end of a wand

Crucio: unforgivable curse, causes extreme pain.

Deletruis: stops Prior Incanto

Densaugeo: makes someone's front teeth grow at an alarming rate

Diffindo: makes a bag split

Engorgio: makes something bigger

Ennervate: un-stuns someone

Expecto Patronum: makes a person be safe when near dementors and forms a Patronus

Expelliarmus: disarms someone

Ferula: makes a splint

Finite Incantatem: stop spells

Flipendo: Pushes objects (in PC game 1)

Furnunculus: make boils appear on other people

Impedimenta: slows down and obstructs attackers
Thanks to Amayas for the spelling correction.

Imperio: unforgivable curse, total control curse

Impervius: makes something repel water

Incendio: starts a fire

Locomotor Mortis: makes somebody be leg-locked

Lumos: makes light

Mobiliarbus: moves trees (mobili means "to move", and arbus meaning "tree")
Thanks to Robert for the correction.

Mobilicorpus: moves a body
Thanks to Amayas for the correction.

Morsmordre: conjours the Dark Mark

Nox: stops Lumos

Obliviate: makes you lose your memory

Orchideous: makes flowers burst from a wand

Peskipiksi Pesternomi: stops pixies - however, this spell was used unsuccessfully by Lockhart, so we can assume that it does not work.
Thanks to Ninetails 3000 (John) for sending this spell in and Skahducky for the Lockhart reminder.

Petrificus Totalus: makes somebody be body binded

Prior Incantatem: show prior incantion done with wand

Quietus: stops Sonorus

Reducio: stops Engorgio

Reducto: blasts solid objects out of a person's way

Relashio: sends sparks at grindylows

Reparo: repairs things

Rictusempra: tickling charm

Riddikulus: makes a boggart become a funny thing

Serpensortia: makes a snake come

Sonorus: makes one's voice magnified

Stupefy: stuns a person

Tanrantallegra: makes you start dancing

Verdimillious: makes you see invisible things (From Harry Potter Play Station game)
Thanks to Ninetails 3000 (John) for sending this spell in.

Vermillious: hurts non-magical animals (from Harry Potter Game Boy Color Game)
Thanks to Nine Tails 3000 (John) for sending this spell in.

Waddiwasi: makes objects fly up someone's nose.
Thanks to Stacy and Chris for sending this spell in.

Wingardium Leviosa: makes things levitate