Mis Viajes Low Cost Spain and Europe (written in Spanish)
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HelloWings Expert on low cost carriers. 365 day chart to find cheapest price.
All The Flight Deals Deals and error fares (You can also sign up for alerts from your airport!)
Secret Flying Shows error fares & deals
Matt’s Flights A great site/newsletter that sends error fares/cheap flights/cruise deals
Skiplagged Find cheaper flights by buying a flight with a layover in the city you're actually trying to go to (hidden-city fares)