This is a dungeon for 1st-level players I ran this summer.
Here are the main bits--I might post more about the details later if I feel like it:
The players wake up in a library, in a pile of corpses. This is the middle of the dungeon.
(This is the best possible beginning for an adventure in my opinion. The only bad thing about it is it starts to get old if you use it more than once a year.)
There's obviously just been a battle, but the players can't remember anything.
This dungeon is built around a chamber containing a huge, fat, (three storeys tall, and three storeys wide) imprisoned demon. The demon way outclasses the PCs but, luckily, it's imprisoned and asleep.
The north--and upstairs--of the dungeon is dominated by a halfling vampire queen (with a pet vampire monkey) and her army of albino cannibal mutant halfling subjects.
The southeast (and downstairs) of the dungeon is the domain of a small group of goblins who've been trapped down there for weeks and are just trying to get out.
The southwest of the dungeon is the opulent and statue-adorned lair of a lonely medusa and her pets. One of her statues is the petrified goblin leader--an alchemist--who the goblins suspect holds the key to getting them out of the dungeon.
The alchemist's lab contains all kinds of useful things that low-level players might be able to use to defeat a medusa or a vampire but they're labelled in an obscure orcish dialect.
In the lowest depths of the eastern part of the dungeon (accessible from the medusa's lair and the halfling section), there's a sphinx who's been imprisoned by the vampire queen. If the PCs free her and/or help her take revenge on the vampire queen, she can provide all kinds of useful information.
Careful questioning of the dungeon's inhabitants and or eating some of the local obliviax will allow the PCs to remember they were part of a large posse/invasion force sent to eliminate the evil halflings, who have been ravaging local cities and towns every night and disappearing before dawn. Although the PCs still won't be able to remember exactly how they got down here.
When the PCs arrive, fragments of the posse will still be battling room-to-room with the halflings.
(The posse will keep fighting in the dungeon for as long as it's interesting, but they'll never get all the way to the vampire queen's inner chambers. They're not as smart as the PCs.)
The PCs won't wake up with all their equipment, but there will be plenty of dead bodies of both sides to loot in the corridors.
The battle in the dungeon has lasted several days, and the rotting corpses lying around have brought all manner of cannibalistic wandering monsters skulking into the corridors.
One PC will wake up with a note from his or her mother (a witch) reminding him or her of a spell she put on him before she let him/her run out and join the militia: "Remember, strangers will always believe the first three words you say."
Killing the medusa will de-petrify all of her victims. It's also worth noting, however, that this medusa is ancient, and a great deal of the dungeon itself is made of stone block cut from the bodies of her frozen victims--such as the imprisoned demon's colossal brothers, and perhaps a few dead gods.
Therefore, killing the medusa may make some corridors unstable (as the keystones turn into inert slabs of geometric flesh), create new ones, and--perhaps worst of all--will free terrible and hidden things that have been walled up in stone prisons for centuries.
So...have fun with that.