It really depends on where you’re travelling from, and where you need to go. Sidestep and Kayak and the USA-based sites are going to miss a lot of offers if you don’t need to travel within, or from, the USA. This forum is read around the world, so without wasting a guess, please edit your question to add this key info. I’ll check back later and try to post something helpful to your specific route(s).

http://www.sidestep.com/. WIth this site you can search every airlines simultaneously with a nice user interface.