Rooms for as low as $2 a night can be booked on Hostels Dot Com.

Angkor Aseana Residence: Siem Reap Vihear Chen, Svay Dankum, House 602. Facebook: gallery and info here.
Basic rooms can be rented for US$60 a month (single) and $75 for two people.
The basic room comprises 20 square meters with private shower and toilet, double wardrobe, small cabinet, bedding and curtains, two salon chairs and table, plus fan. For more creature comforts, pay more money!

Bou Savy Guesthouse: . Website: website here.
US$12 for air/conditioned single with cable/satellite TV; at the moment if you can book online you can receive one free night. Use the Internet downstairs to save money!

Garden Village Guesthouse: . Website: website here.
The US$1 dorm beds here are said to be the cheapest in Siem Reap!

Green Park Guesthouse: . Website: website here.
Rooms with fan, satellite TV and wifi for only $6 a night. Can be booked on their site.

Green Town Guesthouse: Street No 21,#0182, Wat Bo Village, Salakamroeuk Commune, Siem Reap. Website: website here.
Set back off the main road and is only several minutes walk from the center of Siem Reap. Fan singles for US$8 a night, air-con singles for US14. Siem Reap Rooms: . Website: website here.
Room from $6 a night, with free Internet in the lobby downstairs. Can book rooms on their site.