Skyscanner

Skyscanner compares flight prices across airlines and booking agencies rather than selling tickets itself, then hands you off to whoever is cheapest.

Its most useful features are the flexible searches: whole-month price calendars and "everywhere" destination search, which are how you find that shifting travel by two days halves the fare.

Because booking happens on a third-party site, service quality and change policies depend on that agency rather than on Skyscanner — worth weighing when the cheapest option is an unfamiliar reseller.