Trap builders - who are they?

I have a few thoughts on the subject

The Kobolds.
I mean, no brainer, these guys are born trap makers, and whenever they show up in my games, the players are on alert for darts, arrows, spears and spikes tipped with filth rot goo.
Kobolds are frail and sneaky, they like to work for dragons and they just make traps as a matter of course.. its what they do.
They also get into fights with rival tribes a lot, which means that you can run into snare pits of random doom just about anywhere, thanks to these little trap litter bugs.

Tribal Humanoids.
Snares and pits are a staple hunting method of tribal folks, so it makes sense that these will be encountered in their hunting grounds, and around their settlements, particularly areas where they don't patrol much and want to 'set and forget'.

The Lich.
Classic undead hermits, the Lich wants to settle down for a couple of centuries and catch up on its arcane reading and research, so it will find a nice, comfortably morbid lair and trap the hell out of it, stock it with relatively self sufficient guard beasts and cleaning Oozes, then seal itself off and get to work. In my game, the Lich will rarely rouse from it's chambers unless actively disturbed or threatened by meddlesome adventurers, and ideally, will send arcane means to spy on and perhaps threaten the players long before they get to the heart of its lair (and by the time they get there, the Lich is probably already off and planning its revenge).

The Dragon.
The lair lords, a dragon is a classic grumpy and self serving hoarder, and they really hate kids on their lawn.. they employ other creatures to do most of the work for them, be it kobolds, lizard folk, dragonborn, constructs, cultists.. you name it, there is no shortage of lackeys for a great wyrm. If I were a smart dragon (and most of them are pretty cunning), I would stock the lair with traps that I am immune or at least resistant to, and you know all those cool magical items in my hoard? I USE THEM.
Generally a dragon can fly, so chasms are nice. A dragon can set off explosives from across the cavern with a breath weapon, so really, the claws and fangs are final resort weapons against intruders.. unless the dragon is really, really angry, of course.

Cultists.
Neverending cultists.. so much fun to throw at the players because they are crazy and nobody likes them. Cultists have a habit of taking over ruins or the lairs of other creatures to do their voodoo in private, and they will employ traps because they take relatively little effort to maintain, and they can always sacrifice a trap artisan rather than paying him for his services.
Cultists will favor any sort of trap that fits their particular kind of crazy.. poison, acid, strangulation, flesh eating insect swarms, hallucinogenic drugs.. they probably don't have the resources for magical traps just yet, having blown all their money on fancy hats and candles, but you never know, there may be a capable arcanist among them.
Of course, the truly sadistic cults will try to capture their interlopers alive, so they can do extremely nasty things to them later (while gloating, making bwuahaha noises, etc).

Rules of Thumb.
A creature that wants to eat you alive is going to try and capture you alive (eg, mind flayers), a creature that wants to be left alone is just going to try and get rid of you (pit trap full of green slime, carnivorous plants, swarms, flesh eating undead), and creatures that eat the dead will want a nice corpse to pick over afterwards (blades, spikes, accessible pits, etc).
Just because a creature is not smart enough to make a trap, doesn't mean it can't figure out how to use one it finds.

And most importantly.. getting instantly killed by an unavoidable trap sucks, but having an hand lopped off is always fun.

Most Memorable Trap.
The party was enchanted by a pack of fey creatures while raiding a big ruin, searching for some lore. They thought everything was cool and joined the fey for a feast, but half way through, with lots of ominous fortitude and will checks, they snapped out of it and realised to their horror, that the player characters were eating their own limbs.
See what I mean? Losing a hand is always fun.
In this case, the friendly fey who helped find the lore and invited them to a meal were the trap, because it was all just an illusion.

I love the fey feast. Brilliant.