If you haven’t tried it out yet, check out Farechase. I started using it last summer when it was still in beta to book flights to California. It has a slick AJAX interface that lets you change your search by all kinds of criteria on the fly.
This service is different from the others, Orbitz, Travelocity, etc. – not just because it’s ajaxified – but also because it sends you directly to the carrier’s website for booking. Basically it’s an information aggregator, not a booking engine. This is much more attractive to the airline industry because they end up receiving the visitor and reservation on their website. Orbitz, on the other hand, charges a booking fee of about $6 AND acts like the travel agent, booking the flight for you – the customer never sees the airline’s website. This is a big paradigm shift in online travel booking, even if people don’t realize it or notice it.
(Also, the “hotels” search should give you a glimpse of the future of real estate online. All dynamic, all responsive to your whims as fast as the technology allows it.)