Check out the aggregators
Comparison-shop dozens of booking engines at once
An aggregator is like a personal shopper for the internet.
It quickly canvasses the databases of dozens of major booking engines and hotel reservations sites to find the best rates on all the rooms available.
Well, usually the best rates, on most of the rooms. I have put the major sites—like HotelsCombined.com and Trivago.com—through their paces, and while they are impressive, they somehow just don't always manage to scrape every last results in their searches, nor always offer the absolute lowest price out there.
They are well worth checking out to see what they offer, but I always dutifully run a direct search on Booking.com—as well as at any specialist booking sites if I'm looking for alternative accommodations—to be sure I'm getting the whole story.