I am often frustrated trying to find a good deal on a flight on-line.
Note: CheapTickets charges a $9 service fee while Yahoo is still free.

But I really like the (fairly) new Yahoo! site. It has a nice AJAX interface too.

Cheap Airfares on Yahoo! FareChaser.

UPDATE – Just booked a flight on CheapTickets.com — not quite as slick as the Yahoo site. But it still got the job done. Found the cheapest, most convenient fare comparing all competitors.