What qualifies you is paying taxes, so basically if you're on a business visa you get it--if there are other visas which allow you to legally work and pay taxes

What qualifies you is paying taxes, so basically if you're on a business visa you get it--if there are other visas which allow you to legally work and pay taxes, they might qualify you as well. They assign you to a particular hospital when you enter the program, and you're entitled to free care at this hospital, if you need a procedure they can't perform, they'll refer you to another government hospital which will also be free. Waiting periods sometimes are long, so it's a good idea to have private insurance as well. Foreigners in Bangkok at the moment all seem to be getting assigned to a hospital in Phrakanong which has a pretty good reputation. You can change your assignment later, I've heard the Police General Hospital on Rama I is very good. I've heard it's also possible to remain in the program after your employment ends if you pay a small monthly fee (maybe $15). There is also a Thai equivalent of the USA's social security program, though the payouts are very modest. Overall it is a good system, you just have to be aware of the limitations of Thai insurance in catastrophic cases (probably I will switch to an expensive expat health insurance plan in the next year or two because of this).