Thailand has a number of cheap domestic airline options. Travelling by plan is normally the quickest way around a country and some of these domestic airlines can provide you with a flight at a very reasonable cost.
It can also be argued that it is probably safer to travel through the skies of Thailand than the roads!
I have flown Thailand Air Asia many times and I struggle to recall a flight that has taken off on time. At this stage I am willing to invest in other options. See Air Asia - Quality Cheap Flights Throughout Asia for further information about Air Asia.
One thing that annoyed me, was I could never find on the Air Asia website a flight map visually showing their routes.
There was also an issue with their website when I tried to book my friend a ticket. The computer system gave me some vague error message and we had no idea whether it had actually made the booking or not. It appeared not and we did not get an email message until well after we tried to make the booking. Turned out the booking did go through, but we had already made another booking. It turned out okay in the end though as they had cancelled the first booking by the time we enquired at the airport.
I also believe all my flights with Air Asia ran late, but then I think all the flights on that holiday were running late.
NokAir is pretty distinctive with their bright yellow colour. I have not flown the NokAir airline myself, but hear they offer a good service. Many people suggest this is the best cheap domestic option for Thailand. I may have to try them on my next Thai trip. Update: I have tried on at least two separate occasions to book flights with NokAir, but due to different reasons their website failed to take my booking. I wonder how much business they have lost due to the poor quality of the NokAir website.
I believe they offer prearranged seating when you are booking online, this can stop the mad rush for seats that you might encounter flying with an open seating airline like Air Asia.
NokAir also offer their Nok Plus, which is a higher fee seat with some perks such as
The website notes that Nok Plus seats are available at only 535 Baht extra (VAT included) on top of economy class fare.
Below are some airlines that are not low fare airlines, but may be worth checking out.
Now One-Two-Gone (as they have gone out of business) was owned by Orient Thai Airlines.
Bangkok Air call themselves a boutique airline. I have used this airline a few times now and they are quite good and you can sometimes find good deals. In some airports they have a free lounge you can use if you show your Bangkok Air ticket. There are snacks and drinks available for free in the lounge. The flights also provide you with snacks.
Phuket Air website did not do much to impress me. They seem to have limited flight options and after a few attempts, I gave up trying to get it to give me a price. I'm not sure how their fares measure up to the others here.
Phuket Air seem to only run routes along Bangkok, Ranong and Yangong. Their flight route map has a number of other options listed for the future. I'll leave it to you to decide about Phuket Air. Funnily enough, they don't seem to do a route to Phuket!
SilkAir is a Singapore Airlines company and might be worth looking at as it flies some Thailand routes.