use skyscanner.net
I'm hooked on this, skyscanner.net looks at loads of websites, and even tries to make joined up itineraries using low-cost, such Ryanair Manchester to Dublin then Aer Lingus Dublin to Boston etc etc, that kind of cleverness. Far better to say "LONDON" for airport rather than LGW or LHR as then skyscanner can find the best deals. Skyscanner also checks the other aggregatgors such as opodo and expedia..
I've got no interest in the website, just use it for comparison shopping.