You'll need at least Honored reputation with Lower City in order to purchase this from Nakodu, the Lower City quartermaster in (where else?) Lower City, Shattrath. Questing for Lower City, or a few runs of normal Sethekk Halls/Auchenai Crypts will get you to Honored easily.
The Crescent Key is an old-world key for Dire Maul North and West, and you'll need a jaunt into the eastern wing (which doesn't have a key requirement) in order to get it. Enter through the normal entrance (not the "back door" located at the pavilion in eastern Feralas, in case you've got a keyed friend coming along with you) and talk to the imp Pusillin, who'll be located to your left after entering. He's initially neutral, but after talking to him, you'll spend a good bit of time chasing him all over the dungeon. Eventually he stops running, turns hostile, and you can kill him for the key (a 100% drop). At 80 this should be a relatively quick excursion. While you're around Dire Maul, don't forget to check the arena for one of the rare spawn mobs.
The Crescent Key will come in handy if you ever find yourself farming Steamwheedle or Shendralar rep, so if you're on the road to 40 Exalted Reputations or (ugh) Insane in the Membrane, make sure you've got it.
Key of Time
You'll need at least Honored reputation with the Keepers of Time in order to purchase this from Alurmi, the KoT quartermaster in the Caverns. A few runs of Escape from Durnholde and/or Black Morass will get you to Honored easily.
Key to the City
Poor Stratholme, doomed to burn for an eternity. Getting this key is well worth your time if you're trying to farm for the Baron mount, and it's a 100% drop off Magistrate Barthilas, a miniboss in the undead portion. He can be found in one of two places: a). in the plaza before the Alonsus Chapel, or: b). to the right of the gates leading to the Slaughterhouse (the building with all the abominations milling around in front).
Shadow Labyrinth Key
You'll need at least one run of Sethekk Halls on either normal or heroic to grab this -- it's located in the chest behind Talon King Ikiss, the final boss there.
The key for Blackrock Depths is one of the most interesting (and helpful) keys to get, and all the more so because you can only pick up the quest for it if you're dead.
Step one: Die. As most classic players can tell you, there's an array of interesting and exciting ways to die in and around Blackrock Mountain. Some of the more celebrated means include falling into the lava, starting a PvP war around the stone, or being the healer (sucker!) in the Blackrock Depths Lyceum. You may wish to experiment if you find these methods too pedestrian. As an aside, if you do jump into the lava in order to kill yourself, make sure you do it where you can easily climb out. Once you retrieve your corpse, you don't want to die again swimming out.
Step two: Run back. Yes, the graveyard is located a hideously far distance from the mountain. Head for the entrance to BRD/port to Molten Core.
Step three: On the way in as a ghost, you'll run across the equally-ghostly Franclorn Forgewright, who'll give you the quest Dark Iron Legacy. There's actually an interesting bit of lore to this quest; Forgewright was the architect of the sprawling kingdom carved into the bowels of the mountain (the dungeon we know as Blackrock Depths), but died in despair over the endless slavery to Ragnaros into which the Dark Iron nation had willed itself. Reading between the lines on his tale, he's annoyed that the weapon that accompanied him through much of his life (the mace Ironfel) is now being toted by his successor, a guy he hates, and he wants you to dispense some punishment from beyond the grave. Truth be told, he's not too clear on exactly what kind of power the mace holds (nor is he all that terribly clear on exactly what Fineous Darkvire's done to merit his hatred), but we're getting a key and some cash out of the deal and it doesn't pay to ask too many questions.
Step four: Get thee to Blackrock Depths, and head for the Hall of Crafting (the ramp with the golems). You'll need to kill Fineous Darkvire (bonus points if you get the little bastard to cough up his shirt) and take Ironfel to the Shrine of Thaurissan.
Step five: Rampage through BRD killing everything in sight in revenge for the many wipes suffered there in your 50's.
Step six: Profit! No, seriously. BRD is pretty lucrative to run even at 80, especially because you can sell off your bag contents in the bar halfway through if you're carrying too much stuff.
Shattered Halls Key
This place used to be hell on earth to tank back in the day. Ah, memories. Anyway, getting this key is considerably easier than it was in BC, and the questline starts with a drop off the NPC Smith Gorlunk in Shadowmoon Valley. He's located in the "orc area" outside Black Temple, i.e. if you're facing the central courtyard of BT, take the path to your left. Up the hill and off to your left before a set of stairs will be a hut and our buddy Gorlunk. Kill him, and get the Primed Key Mold. This will start the short quest series, which takes you back to Honor Hold (for Alliance) or Thrallmar (for Horde), requires you to collect some materials (4 Fel Iron bars, 2 Arcane Dust, and 4 Motes of Fire), and then kill a Fel Reaver (or find a freshly dead one).
A number of classes can solo a Fel Reaver easily at 80, so the main challenge will simply be finding one. The giant metal monstrosities that plagued you all the way through Hellfire leveling will, as of the moment you accept this quest, vanish completely. However, if you're not easily able to solo a Reaver or you just want to wait for one who will eventually path along, park yourself on the road that snakes northwest from the Path of Glory to Thrallmar. A Reaver has a patrol path that makes a visit to the road, and once you see it, aggro it and run like hell for Thrallmar. There's a group of elite NPC's on the outside of Thrallmar's southeastern wall, and if you can keep the Reaver in that area long enough for him to do his Thunder Clap ability and aggro them all, you can sit back and watch them kill it for you. Afterwards, getting the Shattered Halls key is as easy as standing in the middle of the Reaver corpse and right-clicking the Unfired Key Mold.
The Scarlet Key
Likely the first key you'll ever have the opportunity to get as a player, the Scarlet Key is a 100% group drop from the chest located behind Arcanist Doan in the Library wing of the Scarlet Monastery.
I don't know whether it's just my server, but the Scholomance key is easily the least common among the players I know. It's a shame, as this and the BRD key are easily the most interesting to get, although I'll grant that the Scholomance key is significantly more annoying. Anyway, if you're Alliance, the quest for the key with start with Commander Ashlam Valorfirst in Chillwind Point. It's a short quest series:
- Scholomance: Talk to Alchemist Valorfist, also in Chillwind Point.
- Skeletal Fragments: Bring the Alchemist 15 skeletal parts from the skeletons now infesting Andorhal.
- Mold Rhymes With...: You'll need to make a lengthy trip to Gadgetzan, bringing both the skeleton mold and 15g to an NPC there.
- Fire Plume Forged: Head to the center of Un'goro (Fire Plume Ridge) with the unfinished mold and two Thorium bars. As the Wowhead entry here notes, the ramp up the mountain starts at 52, 42, and you'll need to stand on or around 48,48 at the lava lake.
- Araj's Scarab: One more lengthy trip back to Chillwind Point later, and you'll be asked to go kill Araj the Summoner, the elite lich surrounded by cronies in the center of Andorhal. At 80 (or even 70, for some classes) this is easily soloable, but if you're going to get owned, enlist some help.
- The Key to Scholomance: The Alchemist will finally fork over the key.
The Horde questline is essentially the same deal:
- Scholomance: High Executor Derrington at the Bulwark sends you to talk to Apothecary Dithers (also at the Bulwark).
- Skeletal Fragments: Bring the Apothecary 15 skeletal parts from the skeletons now infesting Andorhal.
- Mold Rhymes With...: It's lengthy-trip-to-Gadgetzan time, bringing both the skeleton mold and 15g with you.
- Fire Plume Forged: Identical to the Alliance quest.
- Araj's Scarab: Head back to the Bulwark, and the Apothecary will send you to go kill Araj the Summoner.
- The Key to Scholomance: The Apothecary will finally fork over the key.
The key to Gnomeregan is a 100% drop off the mini-boss Electrocutioner 6000
, and as a 28 elite, he's pretty easily soloable by most classes in their 30's. Less easily soloable in your 30's are the masses of mob packs you'd have to get through to reach him, so you may want to get a group for Gnomeregan (still not the most popular dungeon in the world) or just wait until you've leveled a bit more.
The Violet Hold Key
The easiest of all keys to get, the VH key is simply handed to you by a warden
outside the Hold after you've talked to Rhonin
. The only requirement for this "questline" is to be level 75, so once you've reached that point, talk to Rhonin in the Violet Citadel, and then head for the Violet Hold.