English sites:
www.skyscanner.net
www.cheapflights.co.uk - in my experience, this encounters THE cheapest flights, but some of the airlines you fly on might be considered a bit dodgey
www.cheaptickets.com -
www.lowfareflights.co.uk - low fare flights departing
from the UK
www.lastminute.com
Of course, you might be suprised at the prices the actual airline websites can offer these days...
Spanish sites:
www.chollovuelos.com
www.vueling.com
www.edreams.es
www.viajar.com
www.muchoviaje.com
www.tubillete.com
www.rumbo.es
www.atrapalo.com
www.ebookers.es
www.marsans.es
www.terminala.com