For those who went through the hassle of checking all combination of dates, airlines and destinations in order to find (maybe) the lowest fare... Skyscanner will change your life. http://www.skyscanner.it/ (I don't know why the .net doesn't work anymore...) I put the link in my favorites.
It's a very flexible searching tool. I love the "whole month" searching function that gives you an overview of the lowest fares.
Another interesting feature is the "everywhere" destination. It gives you a selection in ascending price order of the airports you can fly to. If you have a fixed budget it's an interesting option to find out the cheapest flights.
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