FareCompare (http://www.farecompare.com/)

Kayak (http://www.kayak.com/)

Yapta (http://www.yapta.com/) [yapta allows you to track flight prices and save money before and after you buy]

Written by Shengdong Zhao

Shen is an Associate Professor in the Computer Science Department, National University of Singapore (NUS). He is the founding director of the NUS-HCI Lab, specializing in research and innovation in the area of human computer interaction.