Key to Salem's Chest
The Key to Salem's Chest is required to open Dark Cleric Salem's Chest, which is located in a Scarlet Crusade camp at the northern edge of Venomweb Vale in Tirisfal Glades. It has not been known to drop anywhere to date, fueling much speculation.
According to a post on the European WoW forums  the key is not obtainable in-game as of January 2007. The question and the response are:
Xive: Me and I'm sure a lot of other players is wondering whether the Key to Salem's chest which is used to open Dark Cleric Salem's chest in Tirisfal Glades is attainable in-game as of now.
The quest to get it (if there is one) seems to be broken.
An answer would be greatly appreciated and would make us looking for the key very glad as looking for non-existent items isn't very constructive =/
Another GM interaction on the US server "Ravenholdt" revealed the following:
<GM>[Kilijaz] whispers: As it is now the Dark Cleric Salem's Chest does not hold much bearing on anything, but in the future it may play a more important part in the game.
To [Kilijaz]: i see, so for now, it is not something i may interact with?
<GM>[kilijaz] whispers: Let me just say many players have spent a lot of time searching for the Key to Salem's Chest, I would advise you not do the same.
Speculation on Thottbot
A possible hint was posted by "A Natural Shadow" around March 2006 to the thottbot.com entry for the key. It reads:
The lost key is what ur [sic] looking for. Once hidden amongst the rubble of a statue out of place in the great city turned forsaken. Humans will never return for the sounds of the dead ring loud in the halls surrounding the throne room closed from the world. The sense of the unseen bushes you as you pass. The key has been stolen by one man not quite dead, but not quite alive, cursed by the touch of Salem. He lost himself to the world so that only those whom can save him, who would risk their own life to hand him an unseen arm of salvation. The man has been driven mad by his solitude and often speaks to himself to easy the loneliness of eternity torn.
Have you paid attention to the voices that land on deaf ears in the hall of the undead?
11 months later a poster called "Ä Natural Shadow" ("Ä Natural Shadow" not "A Natural Shadow", so it is probably a fake attempting to look like the original poster) posts another riddle:
The lost key you seek? James might aid you on your quest. have u tried the floor underwater, where fishes and murlocs wander? dose it ring a bell when i say throne room? or did you die of boredom? also in death u can find answers. have u talked to the ghost? but the ghost you seek is not to be seen.
Also someone who calls himself "Trust of Whisper" posts a riddle:
Find the being in lustrous sheen, in the pale grey nothing alone! Up in the fingers of lively boughs, Above the calm of this present. Down in the roots of the grey and thin, Below their chaos of the past: staying between these two before the last turn. Lift from the flesh of the earth, the eloquent posture was never praised at all. Rights of the wild in the rough, there is the scar of life. Left alone: A woman and man, calm and chaos. Write to him from Lordaeron, rotting with laughter remember: Be shy and smile softly to easily trick him. A meeting will take place, that has taken place before! Begin in the cavernous tapestry of mortality, knowing forsaken's bright lore. Undead concentration wrought of pure mana... Thrust of whisper on green, hushed body to pull taught and stir. Rushed azure wide arching null, flooding upside-down deep. In the same location but ages, ages past.
It's not clear where these 'hints' came from, nor whether it actually has any bearing on finding the key. Other postings on the page refer to Undercity residents such as cockroach vendor Jeremiah Payson and Apothecary Keever (who mutters to himself as he performs experiments on his captives), also there have been rumours that you have to use you get in Scholomance. It has also been suggested that the chest is a holdover from the beta test of WoW.