For no-nonsense fare comparisons on various airlines between set destinations, Kayak is hard to beat (it also covers hotel and car hire prices). Simple to use and intelligently arranged, it covers both no-frills and with-frills airlines and you can search for flights from differing departure and arrival airports. Fare alerts tell you which dates flights are cheapest on a particular route.
Like that? Try www.skyscanner. net;www.cheapflights.co.uk; www.traveljungle.co.uk