Here's my thoughts on some of their '40 Most Useful Travel Websites', at least, the ones that I have used..
FareCast - Awesome idea (trying to predict airfares and whether they will drop or fall) but not really useful for Calgarians (or any Canadians really) since our city isn't supported.
Kayak - definitely one of the best sites, especially after you have a good idea of your dates/cities. The number of ways you can sort flights by travel time, stopovers, price, etc, is what makes it so powerful.
Also, I love their hotel search, because you can plot all the hotels on a Google map, sort them by price, and figure out what the best deal is based on location.
Orbitz - my go to site for car rental bookings. Their 'Orbitz Refund' guarantee (if someone else books a seat on a plane for the same price they will 'send you a cheque') is bull, and their rules/regs/restrictions guarantee you will never get that cheque.
Travelocity - used to be one of my favorites, back when they had a decent 'flexible date search'. You could search for prices for an entire range of months, including international flights. Now it's just domestic, and even those don't work a lot of the time.
Trip Advisor - AMAZING site. Oh man, I can't tell you how much better this site has made my vacations. Sort the hotels by review rating and look for a good price. I don't think I've had a bad hotel in 4 years thanks to this site. Also, the forums are awesome for trip planning.
Virtual Tourist - used it a few times here and there for travel tips/ideas.
I'm surprised Fare Compare isn't on the list. In my opinion it's one of the most innovative sites, and does a lot of things that I can't find anywhere else.