If you don't have the time or inclination to wade through hundreds of sites, read newsletters, tweets and set up email alerts to get the absolute best fares - there is now a service that does all this for you. It is called Flightfox. The catch - you have to pay upfront but you get a 25% discount on the fee by clicking here.
It will cost you about $25 but they'll refund you that if they don't save you at least that much on what you can find yourself. Best for multistop around the world type stuff than just point to point.
Not free but save you some $$$$