Prasat Preah Vihear is a Khmer Temple located at the top of the Dangkrek Mountains on the border with Thailand in northen Cambodia.
It has recently been accepted as a World Heritage site. The sublime location which affords magnificent views over the Cambodian plains and the fact that it is a very well preserved Angkor-period monument make it a not to be missed destination.
However, the temple is quite hard to get to from Cambodia, which is actually part of the fun and adventure! You can hire a car from Siem Reap and drive there via Anlong Veng. If you leave Siem Reap early in the morning, you can reach the temple in the afternoon and be back in Anlong Veng to spend the night. You must hire a different vehicle to drive up to the temple itself as the 'road' is awful. Do not stray off the main paths and roads as the area is still full of landmines. Be aware that the roads are shocking practically all the way, which is why very few people make the trip from Cambodia, however it's a brilliant experience.
If you fancy something much easier though, you can also access the temple from Thailand. A beautiful paved road leads from Si Saket to the temple known as Prasat Khao Phra Wihan in Thai. You don't need a Cambodian visa, you just pay a small fee.
The advantage of going there from the Cambodian side is that all the Thai tourists will leave the temple around 4-5pm and you will then have the whole place to yourself. Some people have stayed the night up there with the locals.
Whichever way you choose to go, I can guarantee that it won't leave you disappointed.
There has been tension between the Thai and Cambodian governments as both claim the temple as their own. It has recently (oct 2008) been very bad, so make sure you check what the situation is before going.