If you are looking to go out to the airport to meet someone and you want to know if their flight is on time, check out this website before heading off:
http://www2.airportthai.co.th/html/b-sche-r-a.asp
Becareful though, the flight I have to meet tonight is delayed, but the board isn't showing it yet - so you need check about an hour to an hour-and-a-half before expected arrival.