I've been a big fan of www.skyscanner.net for many years, as it lets you specify a "whole month" instead of a single date and show a graph of prices for every day. Can pick the cheapest day that way (although with my wife being a teacher it means we usually end up flying on the expensive days anyway )
However, recently I've started leaning toward www.kayak.co.uk - the tools to refine and vary your search are excellent, and it seems to return the same prices as skyscanner.net.