In the last weeks we did some extensive research on travel websites (a very wise thing to do as a travel start-up ;-) We found two types of websites which really impressed us, because they are technologically leading-edge.
The first type of website is meta-search. These sites compare and filter the prices of flights and hotel accommodation from different sources. They search a whole bunch of websites, like online travel agencies, airlines, etc. and save you a lot of time in doing this job yourself when you are looking for the cheapest rates. The market leader in this area is Kayak. Other US-based meta -search sites which are worth taking a look at are Farecast, Mobissimo and insidetrip. For German/European travel it is also useful to go to Travel IQ, a German meta-search site.
The second kind of travel websites that we thought are remarkable are trip-planing sites. These sites give you the chance to organize your trip day by day, and get useful hints through the trips which were planned by other users. Two of these sites are TripWiser and home&abroad. Unfortunately both sites are still focused on the US, but with more information on European cities coming in, they offer a nice tool to plan your next trip.