All six international border crossings are open 7 AM to 8 PM and all have visa-on-arrival facilities at the Cambodian immigration checkpoints.
On the Thai side, there are good sealed roads to all 6 crossings. There are no direct bus services from Thailand that cross the border, but there are buses to all crossings except Chong Sa-Ngam.
On the Cambodian side, more basic and less frequent bus services are available from Poipet and Koh Kong.
To Siem Reap & Battambang - see the Aranyaprathet and Poipet articles for more details
The busiest land crossing into Cambodia, and the most convenient for onward travel to Siem Reap and the Angkor Archaeological Park. Long the stuff of nightmares, the road has finally been paved all the way from the border to Sieam Reap and Phnom Penh.
To Koh Kong & Sihanoukville - see the Hat Lek and Koh Kong articles for more details
Take a bus to Trat (transfer in Chanthaburi if necessary) and from there a minibus to the border. After crossing into Cambodia there are two possibilities - the once-daily boat to Sihanoukville (can be unpleasant in adverse weather), or else a minibus or taxi to either Sihanoukville or Phnom Penh.
Those with a yen for adventure can try out the four other international crossings:
Chong Sa-Ngam, Si Saket Province - Anlong Veng
Chong Jom, Surin Province - O'Smach
To Siem Reap- Take the minibus from Surin bus station to the border. Once you cross the border, you are in the middle of nowhere,and there are not currently any buses direct from the border. A taxi can be hired for the 5 hour journey to Siem Reap, most of it very bumpy. However you will get a great and unique insight into rural Cambodian life away from tourist areas. Be warned, this journey can be difficult during the rainy season.
Ban Pakard, Chanthaburi Province - Phra Prom, near Pailin
Ban Leam, Chanthaburi Province - Daun Lem, the geographically closest crossing to Battambang
Ho Chi Minh City - Phnom Penh
Through bus tickets cost ~USD$10-12. Bus passengers must walk between the Cambodian and Vietnamese checkpoints and transfer between the two buses that travel the Cambodian and Vietnamese legs respectively. At this crossing only one passport photo is required for a Cambodian visa.
Alternatively, it can be interesting to join a Mekong Delta tour (2-3 days) that travels between the two cities; the total cost is about US$25-35 for three days.
Ho Chi Minh City - Siem Reap
Mai Linh Taxi Company www.mailinh.vn in Ho Chi Minh City provides bus from Ho Chi Minh City to Siem Reap directly from beginning of 2007. The price is US$18.
Ho Chi Minh City - Kampong Cham
It is possible to cross at the Trapeang Phlong - Xa Mat border but as of now you have to organise your own transport.
Pleiku - Banlung
It is now possible to cross the border at O Yadaw near Pleiku. Cambodian visas are available on arrival. You can get a motorbike taxi to the border from Duc Co on the Vietnamese side, but there is no regular transport available on the Cambodian side. It maybe possible to hitch a ride with the border officials, though.
Voeung Kam, Laos - Dom Kralor, Cambodia
Onward transportation not regularly available. Cambodian visas available, but expect 1$ to 2$ of transaction fees on both sides of the border. Travel agencies on both sides have border crossing packages.