In Thai these places are called โรงแรมม่านรูด (rohng raem mâan rôot) = motel, word by word: "hotel curtain draw" because many rooms are drive-in and the parking is separated from the aisle by a curtain. ม่านรูด can also be applied to people who frequent such places.
Niagara Hotel Sathorn Soi 10 or Silom Soi 9, no personal experience. 2 h for 300, a friend stays on an upper floor (6500 per month with aircon and balcony, the rate of 900 per day they quote at reception is very expensive considering the low standard) and says there is high activity at night.
Suriwongse Hotel near Jupiter 2002, no personal experience.
Update 18oct2016: in 2015, I passed and noticed new name and seems to have undergone renovation (impression from outside, didn't enter).
Update Aug 2015:
Went again on suggestion of a boy and was pleasantly surprised. Rooms renovated, look great (hot shower, flat screen TV with regular program and straight porn) and are good value for 260 Baht. I had just showered when it knocked on the door and staff wanted my boy's ID. He didn't have his ID with him, no problem. (Boy is and looks over 20 years old.)
Thung Maha Mek Privacy Hotel โรงแรมทุ่งมหาเมฆ ไปรเวซี in Soi Ngam Duplee ซอย งามดูพลี about 100 m south of Malaysia Hotel. Googlemaps Full night 650, short time 280 (but only for heterosexual couples). I have been there once (and had to pay 650), full story here with pictures: http://www.gaythailand.com/forums/topic/8478-bangkok-cheap-accommodation/
There are several short time hotels near Nana Plaza (Sukhumvit Soi 4) for 300 Baht (information from internet).
Drive Inn Hotel ไดร์ฟ-อิน โฮเต็ล Ladprao Soi 128/2 ซอย ลาดพร้าว 128/2, 310 Baht 3 hours, 550 Baht overnight (blanket will be provided in that case). Googlemaps: Drive Inn Hotel As the name suggests, several floor of drive-in rooms. Once your car is parked, staff will draw the curtain and some minutes later knock on door to collect fee and give you remote control for aircon. Rooms are decorated car-themed (other short time hotels have rather spartan rooms).
For sake of completeness, I have been to Wiang Samran Hotel โรงแรมเวียงสําราญ, about 200 m south of Khao San road and directly behind Sake สาเก gay disco. Threesome with ladyboy and boy from Saranrom, don't remember price, she paid. Googlemaps: Wiang Samran Hotel
With another ladyboy and the same boy from Saranrom, I have been to a hotel across the river, but don't remember name or location.
Update 21.12.2014 Short time hotels in Pattaya and Chiang Mai are not worth an extra post.
My one experience in Pattaya is here:
I have never used a short time hotel in Chiang Mai, but in a recent thread on sgt there is an overview: http://sawatdeenetwork.com/forum/the-time-movement-chiang-mai-gay-friendly-t31112.html