Someone suggested that we start this over here, so proceed. Give the name of the spell, its effect, the word(s) used to cast it, the language it's in (almost always Latin), and what it translates into in English.
The Killing Curse: Avada Kedavra. (Aramaic, "Die as I have spoken.") Instant death to everyone it hits... well, almost.
The Imperius Curse: Imperio. Puts people under the complete control of whoever casts it.
The Cruciatus Curse: Crucio. The most horrible pain imaginable. Non-lethal, but can cause victims to be driven insane.
The Reductor Charm: Reducto. Blasts solid objects away.
The Disarming Charm: Expelliarmus. Causes those it hits to drop whatever they're holding. Works on anyone, even Lord Voldemort.
The Summoning Charm: Accio. Causes things to fly towards the spell-caster.
The Patronus Charm: Expecto Patronum. When done correctly, causes a giant silver apparition known as a Patronus to emerge from the wielder's wand. This giant thing will bowl over anything it might come in contact with and is the only thing that can drive off a dementor.
The Flame-Freezing Charm: We're never told the activation word, but apparently it causes witches and wizards to feel only the mildest tickle from a flame.
Unknown: Sonorous. Amplifies the user's voice.
The Fidelius Charm: We're never told the word used to cast it. Conceals a secret inside a single living soul. As long as the spell is in effect, the secret cannot be discovered by anyone unless the Secret-Keeper wishes it.
The Reverse Spell Effect: Priori Incantatem. Reveals the last spell used by a wand. Also, when two wands with the same core are forced to do battle, this effect will occur on one or the other, forcing it to reveal every spell it's ever done as long as the two are linked.
The Stunning Charm: Stupefy. Causes temporary unconsciousness. Can be deflected by armor.
The Bubble-Head Charm: Unknown activation word. Causes a bubble of air to be maintained around the user's head in any environment.
The Jelly-Legs Jinx: Unknown casting words. Makes the target's legs wobbly and useless.
The Cheering Charm: Unknown casting words. Makes the target happy.
The Shield Charm: I don't remember offhand if there's a casting word. Creates a shield around the user's body which can block some magical attacks.
The Leg-Locker Curse: Unknown casting words. Causes the target's legs to become immobile.
The Body-Block Curse: I know Hermione said a word to cast it, but I don't know what it is! Anyway, the target's entire body is rendered immobile.
I think that's about enough out of me right now.
The Stunning Charm is "Stupor", at least in the German release.
The Body Block Curse: "Petrificus Totalus". Used by Hermione in the first book.
The Door-opening Charm (Book 3): "Alohomora": Opens doors which are not magically locked.
Unknown: "Avis" (4) (latin: Birds): Some birds emerge from the wand.
Unknown: "Deletrius" (4): Makes the Prior Incantatem charm invisible.
unknown: "Densaugeo" (4, Latin: growing teeth): The victim's teeth grow incredibily long.
unknown: "Diffindo" (4): damages a pocket, so that the contents fall out.
unknown: "enervate" (4): Counter-Charm to Stupor
unknown: "Engorgio" (4) Increases the seize of an object or animal
unknown: "Furunculus" (4): Makes furuncles grow in the victim's face.
unknown: "Impedimenta" (4): Slows down the victim
unknown: "Lumos": Creates a Torch flame on the wand
Unknown: "Mobilcorpus" (3) (Lat: mobile body): Levitates an unsciosus man.
Unknown: "Nox" (Lat. Night): Switches off the magic Torch
The Dark Sign (4) (Correct Vocab? German: Das Dunkle Mal): Morsmordre: Voldemorts Dark Sign emerges off the wand.
Oblivate (4): Helps somebody to remember.
Reducio (4): Counter-Charm to Engorgio; Lat: to reduce
Reparo (4): Repairs broken glass. (Lat: to repair)
Rictusempra (2): Sorry, what is the Germen: "Kitzelfluch" in English?
Quietus (4): (Lat: quiet) Counter-Charm to Sonorus.
Um, the Boggart Death Charm: Riddikulus. Causes a boggart to shift shape from something scary to something hilarious. If enough people can do this in succession, the boggart will get confused and ultimately be destroyed.
Orchidaeus (4): Grows some Flowers (orchids?) out of the wand.
I think there's one of these at www.geocities.com/darkcyradis
I believe that "Oblivate" causes a person to forget things. It's Gilderoy Lockheart's favorite spell. I think that it derives from "oblivion".
I think, "Oblivate" was also used during (or shortly before?) the Final Game of the Quiddich World Championship.
Obliviate is the Memory Charm. It was used to erase the memory of the Muggle Camp owner.
Padawn: ... www.geocities.com/darkcyradis
This link does not longer work. Has this site moved to a new adress, or is it died?
Unknown: "Mobilcorpus" (3) (Lat: mobile body): Levitates an unconscious man.
Luigi Novi: Actually, it levitates anyone whose legs don't work.
Reparo (4): Repairs broken glass.
Luigi Novi: It's not limited to glass. Arthur Weasley used it in Chamber of Secrets to repair Harry's glasses, which broke in two at the bridge.
Guys, if you want a reference guide, use the Harry Potter Dictionary, one of the resources at Scholastic's Official site, which you can find here.
Thx for the link.
BTW: Another spells from book 2:
Serpensortia (2), Lat: Serpens = snake. Summons a snake. Seems to be a favourite spell of the Slytherins. ;)
Tarantallegra! (2) forces the legs to dance a fast Foxtrott.
Finite incantatem! (2), lat: finish the spells! Stopps minor curses like rictusempra and tarantallegra.
ANONYMOUS: Rictusempra - "Kitzelfluch" translates as "tickling curse", though I think the actual spell is referred to as the "tickling charm".
NOBODY has mentioned "Wingardium leviosa" so far? Swish and flick! Swish and flick!
Makes objects levitate (derived from "wings", and "levitate", I think.)
There is a spell used by Hermione in first year in order to inconvenience the Devil's snare. She conjures up sunlight and I think the spell is called "Solaris", but I'm not sure.
"Tarantallegra" is derived from an Italian dance called the "Tarantella", I think.
The name of the blue flame spell and the chant used were never mentioned, netrat.
Not even in the book? I haven't got the books with me so I can't check it.
What, you don't believe me? 
I checked it. She used it first to distract Snape during the Quidditch match when she thought he was the one hexing Harry's broom, and then again to escape the Devil's Snare plant. In neither case did Rowling mention the name of the spell, or the chant.
Waddiwasi! - Lupin used it against Peeves who had put a cewing gum into a keyhole. The spell caused the chewing gum flying out of the keyhole directly to Peeves' nose.
Sorry, I don't know the meaning of the word.
Evulsio Tuus Crus - Used by Snape to kill Dumbledor in book 5.
Merat, you BETTER be joking.
Translate it, Luigi. Its dog-Latin, by the way, so the grammer is lousy.
I can't translate it. I don't speak Latin, and none of the online reference sources I could find worked.
It means "Pull(ing) your leg"
If you just say something in Latin and it happens as a spell, what did wizards do in Roman times?
'Good grief! What a long march! I think my legs are going to fall off!'
(CLUNK)
'••••-us!'
Actually, how about 'Inanime Functocherie', with an acute accent on the last e of Inanime? I'm not entirely sure what it means - I just made it up - but it sounds appropriate for a killing curse.
Here come some real spells out of Book 5:
Pack! - Packs the trunk.
Scourgify! - a cleaning spell
Locomotor trunk! - a useful spell to move a heavy trunk.
Disillusionment charm: Makes somebody indistinguishable from the background. Must be countercast if the person wants to become visible again.
Evanesco! - makes the target vanish.
Silencio! - makes the target to be quiet.
Portus! - Changes an item to a portkey.
Legillimens! - a mindreading charm.
Colloportus! - closes a door.
Protean Charm - used to make things grow hot when one of hte objects are trigered, used by Voldemort to gather the Death Eaters, and by Dumbledore's army to notify when the D.A. will meet next...