To be sure you will get the lowest fare possible, you sometime have to turn to different scanners on the market and compare the results. The reason for this is that different scanners work with different airlines and sometimes one scanner can give you a better price from another airline which is not covered by the other scanner.
There is a whole sea of scanners on the market and there are far many then the ones that will be listed here below.
But it will give you a little bit more advantage in your hunt for great deals!
www.expedia.com (also available as country specific with .de .co.uk and so on)
matrix.itasoftware.com - good for intercontinental flights
www.skychecker.com – pulls info direct from Amadeus
www.skyscanner.net - good for flights within Europe
www.flightfox.com – Let others search for your ticket
Tips on how to search for tickets you can find in this previous posts
Which day is cheapest to buy tickets on?
Booking flights for national holidays
Let someone else find your flight tickets
A new flight scanner on the move