Spirit Airline is a newcomer to Jamaica. They only fly from Ft. Lauderdale and Orlando, but they're running a great special for Feb, I just found out: US$20 each way to Kingston or Montego Bay.
Here's the link with details, dates, etc:
http://www.spiritair.com/welcome.aspx?pg=salesinformation&number=127
Some more specials for Feb/March
http://www.spiritair.com/welcome.aspx?pg=salesinformation&number=129
Looks great but do not forget all the taxes and add $125 on for international taxes Than add up and see if it is still a great deal