Kayak is travel search supreme. I think it’s a great example of how well search can work to deliver on intuitive user needs. It’s a travel search specialist that aggregates content from about 120 other travel sites, compares and finds the best for your needs. Technically, I don’t think what they do is tricky, but there is a lot of in page searching for a fantastic search experience. All the hygiene factors that turn me off most airline sites or vliegtickets.nl variants are removed here.
In page, you can choose to not allow 1-stop or 2-stop flights, select or remove certain airlines, choose leaving or return take-off or landing time ranges, airports, flight duration and price ranges.
Once you select a flight, it re-directs you to the partner in question. They make money through PPC and SEA.
They have some nice slightly experimental extensions, such as **dgets, mobile search, APIs for mashups and RSS available through their labs.