Need a dungeon right now? Grab a sheet of paper.
Throw down five dice.
Draw a room around each die that landed on the paper. Throw away those dice that fell onto the table.
Put some big nasty monster in each of these rooms. Something that's going to lair up and not go wandering around too much.
Now, for each room look at the pips on the die*. Add some rooms around it until it looks like the pips. So a 4 is a box, 5 make a sort of cross. You get the idea. Connect the surrounding rooms to the monster lairs, but keep each monster's territory separate for now!
Now in each rooms put something linked to the nearby monster. A home for their minions, their treasure hoard, the place they take a dump. Something to warn adventurers that they're getting close.
Now we want these territories to be connected, but buffered. Put in some rooms that would act as a barrier. Could be through sheer size (a sprawling sewer) or being difficult to cross (gaping chasm). You could even put something nasty in there that nobody would want to wade through (worm nest). Use these neutral rooms to connect the monster territories.
Look around for the least horrible parts of the dungeon. Attach some entrance points to the corridors around there and you're finished!
Oh, and if you're feeling generous add some treasure. You can see I didn't bother here. Adventure!
*Yes, this is where I somehow knocked the yellow die from 3 to 4 without realising.