Google flight

An innovation of Google Flights is that it allows open-ended searches based on criteria other than destination; for example, a user may search for flights within a range of times and a budget and be offered various destination choices. Alternatively, a user can select a destination, and Google Flights will calculate every price for each day of the next 12 months, visualized in a graph or table. This allows users to easily spot a cheapest date to fly to the destination.

http://www.google.com/flights/