I too use kayak, but I also like airfarewatchdog.com. You can get emails telling you the best fares from your city, or you can check the website and type in the city/airport you want to check. I have found that sometimes a great rate is posted on one end but not the other, even though available both ways, so it's best to check both the airport you're flying into and the one you're flying out of. They often have deals that aren't listed elsewhere and last-minute fares, but you usually have to grab it when you see it.