According to Forbes.com, they are (drumroll, please):
BreezeNet.com, for rental car info
Concierge.com, for info from Conde Nast Traveler
FlyerTalk.com, for everything airline-related
GridSkipper.com, for the inside dirt on cities
HotelChatter.com, for the inside dirt on hotels
igougo.com, for trip inspiration and what to do in various places
Kayak.com, for online booking of hotel, airfare, etc.
, a compendium of ways to search travel blogs
Priceline.com, for online booking, especially for hotels
SeatGuru.com, for the lowdown on airline seating on all types of planes
Site59.com, for last-minute bookings
TravelZoo.com, with great deals for flexible travelers
VijayDandapani.com, an inside the hospitality industry blog by an industry insider
I'm glad a lot of my favorites made the Forbes list! I'll have to check out that last one—that's a new one to me.