Well one website did an investigation into this, and found:
Garden Village Guesthouse in Siem Reap, Cambodia
Which has beds for US$1 per night.
To quote:
But, there’s a catch…
The $1 dorm beds are in a thatched-roof hut that is partially open to
the elements. It appears that you do get a mattress, pillow, towel,
and even a mosquito net, but that’s it. There’s also no proper locker
system out there so you have to leave your valuables at reception when
you are away.
Nighttime average low temperatures are steady, ranging from 79F/26C in
March to 68F/20C in January, and it’s always humid, so it might not be
too comfortable on the warmest nights, but at least you’ll never be
cold.
Also, the dorm beds are right next to the kitchen, so you need to be
prepared for some noise and smells while you are saving all that
money.
On the other hand, the Garden Village offers:
Free pickup from the airport or bus terminal
Free (slow) Wi-Fi in the public areas
A volleyball court
Laundry service
Cheap bicycle rental
Affordable Khmer restaurant
Rooftop bar with cheap beer
So you really do get most of the normal benefits of staying at a
hostel, even for this ridiculously low price. Want something better,
Mr. Moneybags?
If you can afford US$6 per night you can get a “standard single” room
of your own, ranging up to a “deluxe triple” room for US$13.
You can’t book the $1 dorms online, but you can book all the other
private rooms from their website. I’m told you can reserve the $1
dorms by emailing them, so you won’t have to just roll the dice and
hope one is open.
Lest you think the Garden Village is just trying to get people stuck
in some remote place to sell them food and drinks, it is actually very
well located, only a few blocks from Pub Street and the main tourist
district of Siem Reap. Also, they have a good and cheap restaurant,
plus 50-cent Angkor draft beers available all day at their sunset
rooftop terrace.