If you'd rather not spend hours looking for the absolute best deal on your next flight, Flightfox may be worth your while. The service's "flight hackers" compete to find you the lowest possible fare in return for a $US29 finder's fee (refundable if you're not satisfied).
Currently, the average savings listed over base flight prices is $US373, but if you browse past savings you'll see that international and multi-city flights are especially ripe with possible savings if you let someone else do the searching for you rather than going the traditional Kayak or Expedia route: In a Canberra to San Francisco contest, the price saved was $US2252 per person (24 per cent), and a flight from Singapore to Hartford resulted in $US1203 saved per person (54 per cent).
Flightfox has a few things going for it. As mentioned, the fee is entirely refundable if you don't get the savings you want. Second, you can add in unusual requirements for your trip, such as you're travelling with three cats, or one person travelling with you will be flying back at a different time, or you demand a TV screen in front of you for the whole trip. It's an alternative to using a travel agent, and the fee covers each person/ticket on the trip (rather than having to pay a fee per ticket).
Worth a consideration, at least, if you have a big flight coming up and don't want to spend hours shopping for it.