Passports holders from Brunei, Indonesia, Laos, Malaysia, Myanmar, Phillipines, Singapore Thailand and Vietnam can visit without a visa.
You may obtain your visa upon arrival at the two airports in Cambodia: Siem Reap and Phnom Penh, along with most border crossings. You will need US$30 cash for a tourist visa and US$35 for business. You will also need one passport size photo and your passport must be valid for six month from the date of entry. Try and have the exact amount or you may be given a very bad exchange rate by the border official. You will need a passport photo or you may have to pay US$2 extra.
A visa can be issued upon arrival at the border checkpoints into Cambodia.
Thailand: Cham Yeam (Koh Kong), Chorm (Oddar Meanchey), Daung (Kamrieng Battambang), Poi Pet (Banteay Meanchey), Prom (Pailin), O Smach (Oddar Meanchey).
Vietnam: Bavet (Svay Rieng), Kaoam Samnor (Kandal Mekong), Phnom Den (Takeo), Trapaing Sre (Kratie).
Laos: Dong Kralo (Stung Treng).
There is also an entry at Sihanoukville if you arrive by sea.
Passport holders from Afghanistan, Algeria, Saudi Arabia, Bangladesh, Iran, Iraq, Nigeria, Pakistan, Sri Lanka and Sudan need a visa at an embassy prior to arriving.
Cambodia visa information
Cambodia has recently started an e-visa service which allows you to pay online using a credit card instead of handling cash at the airport.
The e-visa is only for entry at Phnom Penh International airport, Siem Reap International airport, Bavet/Svay Rieng, Cham Yeam/Koh Kong and Poipet. It costs US$7 dollars more than paying cash.