Compares millions of flights to find you the cheapest deal

I'd also highly recommend Skyscanner! You can filter the results with different criteria, so if you only want direct flights, you can get rid of any that involve transfers. It also lets you search for nearby airports to find the cheapest deal. My only criticism is that Skyscanner doesn't take into account certain fares - for example, SAS does 30% discount for under 26s, but Skyscanner doesn't show this.