Most border crossings open for foreigners, with an indication where visas on arrival can be issued, are listed on the web site of the National Tourism Administration (http://www.tourismlaos.or...). This list is unfortunately incomplete.
Visa on arrival for Laos is now available as of Feb 2010 when entering from Cambodia overland previously was not available, with an official "Visa on Arrival" office incorporated into the checkpoint. The nearest Cambodian town is Stung Treng, and the border is a 90-minute speedboat or bus ride away. Note that the border is lightly used, with almost no onward public transport available at the border therefore book through transport from Stung Treng to Ban Nakasang for Si Phan Don/Don Det and both customs officers and transport providers have a reputation of gouging foreigners, although this seems to have improved recently currently both Cambodian and Laos border officials request US$1 stamp fee per country. Crossing the border Oct 2010 the Cambodia officers will ask for US$1 for exit stamp.You can tell them you don't have any and they will still stamp it. On the Laos side they will demand $2 for entry stamp, if you refuse they will not stamp it, you will need the stamp to get out, so you have no choice than to pay the bribe. Note if you cross the border by boat, you will have to return by road to the border checkpoint to officiate your arrival ie. get your passport stamped in Laos.
Two pitfalls at the Lao-Cambodian border are that you will often have four changes of bus some of them tiny minibuses where passengers have to sit on each others' laps, and hours spent driving to remote guesthouses to pick up backpackers; if your luggage has been sent in a bus you are not on because of 'lack of space' it will sometimes disappear. The 'King of Bus' company is known to do this.
The land crossing between Mengla Yunnan and Boten Laos is open to foreigners and visa on arrival is possible $37 for UK citizens or you can get in advance at the Lao consulate in Kunming. Daily bus service operates from Mengla to Luang Namtha and Udomxai. Buses from Mengla to Luang Namtha leave from the North bus station. The first bus leaves around 8am and costs about 40 RMB.
Generally speaking, it is not possible for independent travellers to cross from China to Laos via the Mekong River, not least because there's a chunk of Myanmar in the middle and the Lao checkpoint at Xieng Kok does not issue visas on arrival. Travel agents in China, including Panda Travel (http://www.chinadam.com), run irregular cruises from Jinghong China via Chiang Saen Thailand to Huay Xai Laos, but schedules are erratic and prices expensive.
Foreigners cannot legally cross the Laos/Myanmar border.
There are Eight border crossings open to all between Thailand and Laos. From north to south:
Huay Xai/Chiang Khong: Fourth bridge under construction. Usual route to/from Luang Prabang, easy bus connections to Chiang Rai and points beyond on the Thai side.
Muang Ngeun/Huay Kon: Visa on arrival. 40 Km from Pak Beng.
Nam Hueng/Tha Li: Easily reached via Loei on the Thai side, but 378 km of dirt road away from Luang Prabang. No visa on arrival.
Vientiane/Nong Khai: The first Friendship Bridge and the busiest of crossing of them all. Direct trains from Bangkok now available.
Paksan/Bueng Kan: No visa on arrival.
Tha Khaek/Nakhon Phanom: Third bridge under construction.
Savannakhet/Mukdahan: The Second Thai-Lao Friendship Bridge.
Vang Tao/Chong Mek: On the route from Pakse to Ubon Ratchathani
There are at least six border crossings that can be used by foreigners. These include:
Donsavanh - Lao Bao - to/from Savannakhet
Keo Nua Pass
Lak Sao - to/from Khammouan Province
Nam Can - to/from Plain of Jars
Na Meo - to/from Sam Neua
Tay Trang - to/from Muang Khua and Nong Khiaw
Bo Y nearest town on Vietnamese side being Ngoc Hoi and on Lao side Attapeu
The international airports at Vientiane and Luang Prabang are served by national carrier Lao Airlines (http://www.laoairlines.com) and a few others, including Thai Airways (http://www.thaiair.com), Bangkok Airways (http://www.bangkokair.com) Luang Prabang only and Vietnam Airlines (http://www.vietnamairlines.com). Some seats on flights of Vietnam Airlines are reserved for Lao Airlines codesharing / better price. Pakse is the third international airport, with flights to/from Siem Reap Vientiane - Pakse - Siem Reap by Lao Airlines and from/to Ho Chi Minh City.
Laos used to be off-limits to low-cost carriers, however AirAsia (http://www.airasia.com) now flies to Vientiane from Kuala Lumpur three times a week. Another cheap option for getting to Vientiane is to fly to Udon Thani in Thailand with discount airlines Nok Air or Air Asia and connect to Nong Khai and the Friendship Bridge via shuttle service directly from the airport 40 minutes; from here Vientiane is just 17 km away.
If you're coming from Chiang Mai, please note that there are no ATM or money changing facilities at the international departure terminal of the airport. If you're planning to get a visa on arrival in Laos, make sure you get enough USD "'in town'" before you leave for the airport. Thai Baht is also accepted, but at very inflated rates.
The long-awaited first link across the Mekong from the Thai town of Nong Khai to Tha Naleng near Vientiane finally opened in 2009. There are two shuttle services per direction per day, with one timed to connect to the night trains to/from Bangkok. Visa on arrival is available when crossing the border by train. The train is not a very attractive option because the railway station is in the middle of nowhere.