There are a few carriers operating BKK-HKT, and others starting soon:-
Air Asia 2 flights per day 1200Baht return
http://www.airasia.com/
Thai Air 7 flights per day 4900Baht return
http://www.thaiairways.com/
Nok Air Starts 2nd quarter 2004
http://www.nokair.com
Bankok Air 5 flights per day 5200Bt return
http://www.bangkokair.com/
Phuket Air 1 flight per day 5200Baht return
http://www.phuketairlines.com/
Air Andaman coming soon!!!!!!!!!!
http://www.airandaman.com/
I normally book through Opodo, and it normally gives me Thai Air, but this time when I did Phuket and Koh Samui I flew Bankok Air and enjoyed it, for your extra Baht you get a dedicated departure lounge with free soft drinks, snacks and internet access.