As far as I know visa on arrival (US$30), is available at all legal international entry points into Laos.
My calculation is that this should be 14 places, but there is some doubt about
1. Tha Khek from Nakhon Phanom in Thailand.
2. Voeun Kham / Dong Crolor, Cambodia. Not strictly legal, but people & a few bikers have crossed here.
Immigration checkpoints where visas can be obtained on arrival are as follows:
(1) Lao-Thai Border:
Houai Sai-Chiang Khong (in Bokeo Province)
Vientiane-Nongkhai (Lao-Thai Friendship Bridge Checkpoint)
Savannakhet-Mukdaharn (in Savannakhet Province)
Vang Tao-Chong Mek (in Champasak Province)
(2) Lao-Vietnamese Border:
Nam Phao or Keo Neua-Kao
(3) Lao-Chinese Border:
Wattay International Airport (Vientiane);
Luang Prabang International Airport ( Luang Prabang Province )
Pakse International Airport ( Pakse Province )
I also saw the Loas web site from the Bangkok embassy, and there's a bit of a contradiction on it, I think. First they say travellers from countries where there is a Loas Embassy or consulate "should" get visas before they leave those countries. Then, almost immediately below, is the list BobS posted above showing the crossing points where visas on arrival are available. I guess what they mean is that they recommend getting visas before travelling if they are available in your country of origin, but, if you don't, they will issue visas on arrival at the four points listed above? I also spoke to someone at the Embassy in Bangkok by phone. He was in the Visa section (extension 106), and he confirmed that visas on arrival were available for US and Thai citizens at the international entry points. So I guess they are available, but there is a bit of a conflict on their web site. Hope this is helpful.
We'll be going to Loas next week, and we'll post again with our experience with the entry/exit paperwork.