Not sure if this has been mentioned elsewhere in this section but the New York Times Frugal Traveller has highlighted a good website which picks up low cost airline flights from anywhere to anywhere in Europe.
It's a little tricky to get used to at first - you can type in any departure country and find the destination by using the map and clicking on it.Rather annoyingly if the country is under the page of flights available you have to keep moving the map bfore you can click on it.