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Swoodoo had a very good approach to this. You could type your departure country and your arrival country and it would query all flights between them.
You could pick a date, it even lists you the cheapest dates and then you could proceed to book your cheap vacation.
Unfortunately Swoodoo now changed the featureset of Powersearch so that you can't put to big locations.
So for example, the following query would go through in the past:
Start: Europe End: World
or
Start: Germany End: World
It had advantage of booking really cheap vacations but now its gone. Anyone know why they did this and why is nobody able to do it? I mean it can't be that hard on the technical site, so are they maybe pressured by airlines?