this company is out of malaysia, but services thailand as well (maybe more?):
air asia. they do super cheap flights, i mean INCREDIBLY cheap. i just booked two one-way tickets from bangkok to phuket for $60! sure beats that 13-hour train ride.
everything seems to be all good... so thought i'd share the love.
another great one, which i haven't had the opportunity to try, but does seem highly respectable as it is a sub-company of thai airways:
nok air. they don't service as many cities, but damn cheap flights even still.