Prasat Preah Vihear (ព្រះវិហារ) is a Khmer (Cambodian) temple stunningly situated atop a 525-meter cliff in the Dângrêk Mountains in Cambodia, right across the border of Si Saket Province in northeastern Thailand.
Preah Vihear is perched on a hilltop with a commanding view of its surroundings. Predating Angkor Wat by 100 years, the history of the temple/fortress is somewhat unclear, but it is known to be dedicated to the god Shiva and thought to have been constructed in the reign of Suryavarman I (1002-50), with further significant additions by Suryavarman II (1113-50). Unlike most Khmer temples, the temple is constructed on a long north-south axis, instead of the usual rectangular plan facing east.
Though easily accessible from present-day Thailand, and for some years occupied by that country, the temple was nonetheless claimed by Cambodia on the basis of a map prepared during French colonial times. In 1959 Cambodia brought the dispute to the International Court of Justice, which in 1962 ruled that, because Thailand had for years accepted this map, Cambodia had sovereignty over Preah Vihear. Soon afterwards Cambodia was plunged into civil war. The temple remained open to the public from Thailand (although unreachable from Cambodia) until 1975, when it was occupied by the Khmer Rouge, whose rusting artillery guns still litter the area. It re-opened from the Thai side in 1998, and in 2003 Cambodia completed the construction of a long-awaited access road allowing Cambodians to visit the temple. In 2008, after a contentious nomination process, the temple was listed as a UNESCO World Heritage Site.
If this is out of your budget, the nearest town of any size is Kantharalak, which can be accessed by frequent public bus in 2 hours or so from the nearby towns of Ubon Ratchathani and Si Saket. For the last leg of the trip (34 km), however, you will have to hitchhike or charter a songthaew/tuk-tuk/moto taxi.
At the entry gate into Khao Phra Wiharn National Park, you will have to pay a 200 baht entry fee (Thais 20 baht); note that the park is open only from 8 AM to 3:30 PM. The road ends at a large parking lot, the final leg (less than a kilometer) into Cambodian territory you will have to cover on foot. At the Thai immigration post you'll be charged an additional 5 baht for a second ticket, and you'll also have to show your passport - they'll take a photocopy, but no stamps are issued and no visas are needed. After the road ends, walk over the smooth rock surface to the entry gate and pay another 200 baht fee (this one to enter Cambodia) and get your ticket punched, and now you can proceed to the ruins.
How to get there: From Bangkok, use highway 1 (Pahol Yothin Rd.) turn right at Saraburi into highway 2 (Mitraphap Rd.). At Amphoe Si Khew, turn right into highway 24, and travel via Amphoe Pak Thongchai, Sangka, and Ku Khan. Turn right into highway 221, and head to Amphoe Kantaralak and keep going to the park.
From Ubon Ratchathani, use highway 2178 and 221 via Amphoe Varin Chamrap, Samrong, Benjalak, and Kantaralak to the park.
A packed laterite access road from Siem Reap via Anlong Veng, a distance of over 200 km, was completed in 2003. Road conditions remain poor and the trip is best done by dirt bike or 4x4. Dancing Roads  regularly arranges multi-day bike trips from Phnom Penh to Preah Vihear.
You can also reach the place on a three day motorbike trip from Kompong Thom.
The only way to get around is on foot. The 500 m elevation and the resulting breeze provide some relief, but it's still a hot and sticky 120 m (vertical) up the hill.
From the Cambodian side, you can hire a motorbike-taxi to take you up the steep ascent to the foot of the temple, but you'll still have to climb up the stairs yourself.
The Thai and Cambodian paths join together at the bottom of the slope (lower end of the adjacent map), and from here the only way is up.
There are several other minor sights in the area, accessible only from the Thai side:
There are ramshackle assemblages of shacks at both the Thai parking lot and the Cambodian base of the hill, as well as all the way along the path up the hill in the temple area itself. These sell not only the expected T-shirts, postcards and cans of Pepsi, but premium cognac and cigarettes by the carton as well: it's tax-free shopping for the Thais! As foreign visitors are few, expected to be besieged by little boys and girls shouting "Hello" and hawking postcards, but they usually take the hint after a couple of "bye-byes".
For more selection and a semblance of hygiene, there are a number of roadside restaurants on the Thai side before the park entrance, along the road from Kantharalak.
Drink stalls are ubiquitous along the trail.
There are only very basic accommodation options in the immediate vicinity.
Cambodia: the village at the foot of the mountain provides two or three very basic "guesthouses" in simple wooden shacks. There is also a wooden, very basic guesthouse (shower and loo outside) at the bottom of the steps, where the locals live. Shower is from a barrel of rain water and a bucket. Very, very basic but very clean. Electricity from 6PM to 10PM.
Thailand: the nearest place with a variety of accommodation is the town of Kantharalak (approx. 30 km), which is also the nearest place with direct bus services to Bangkok, Si Saket, Ubon Ratchathani, etc.
More distant Thai-side possibilities are the towns of Si Saket (approx. 95 km, and nearest train station), and Khu Khan (approx. 95 km, and most convenient place to stay near the border if travelling to/from Anlong Veng); and the city of Ubon Ratchathani (approx 120 km) - however, the most direct access to all these places is via Kantharalak.
If you have your own equipment, there is a campground in the Khao Phra Wiharn National Park. Call the Department of National Park, Wildlife, and Plant Life  at +66-2-562-0760.
Land mines remain a real danger in the area, although the temple itself and the access paths have been painstakingly cleared by the HALO Trust. Stay on the beaten path, don't venture into any vegetation which has not been cleared recently, and heed the red warning signs, painted rocks and strings marking the limits of the demined area.
The cliffs are steep and no provisions are made to protect you from your own carelessness. Keep a very close eye on children.
The Thai tourist centre of Khao Phra Wiharn National Park can be reached at tel. 04-561-9214.