You know the major players in hotel booking and search systems. Expedia, for instance, just bought Orbitz and previously acquired Travelocity, and Priceline owns Booking.com and Rentalcars.com plus Kayak.
But they don't control the entire marketplace — yet. You might find better results with the smaller players. Here are some to consider:
GoSeek.com: This metasearch engine scours the usual suspects (the big agencies), then identifies those hotels that offer coupon, mobile, AAA or specialized promotions for seniors.
Although it displays many deals openly on an initial search, GoSeek also operates on a membership basis, displaying what it calls a "subscriber rate too low to show" for some hotels.
On a sample search for a Boston room for mid-March, it showed promotional deals for 83 of 194 hotels. The displays also post comparison "their rates" figures from the other search systems. Some of the coupon deals are trivial — $1 off a Doubletree Suites hotel isn't exactly a bonanza — but others are substantial.
HotelPower.com: Also a metasearch system, HotelPower requires a paid membership for access to the best deals. A 14-day trial costs $4.94; a year costs $49.95.) Membership provides cash back up to 5% on some bookings and other promotions not openly displayed. When I checked, most of the best deals were prepaid and nonrefundable, but if you're willing to commit, some of the prices were good.
AllTheRooms.com: This meta-search system claims to be the "world's most comprehensive" listing of accommodations. That's unknown, but its roster of 1,251 possibilities in Boston looks like most of what's available. This website searches some obscure booking agencies, such as Hotel.de, based in Frankfurt, Germany. It also includes some Groupon offerings.
Vacasa.com: If you're interested in a vacation rental but are concerned about paying for a property that isn't like the online posting claims, Vacasa can help. Besides arranging rentals, Vacasa is also a professional property manager, which means it monitors and maintains every property it lists.
As is usually the case with managed rentals, you pay more than you would with a rental arranged directly with an owner. A curated site such as Vacasa means you're less likely to encounter unpleasant surprises. Coverage is reasonably good in California, Colorado, Idaho, Oregon, Utah and Washington state, but limited in Florida, Georgia, Hawaii, Maine, Massachusetts, Texas and Vermont. The only foreign listings were a few properties in Belize and Mexico.
Tripping.com: The metasearch system for vacation rentals culls offers from all the big online postings, including several of the giants (HomeAway/VRBO, FlipKey and AirBnb), along with smaller outfits such as Bedycasa.com, Roomorama.com and Wimdu.com. It also picks up a fair number of bed-and-breakfasts, but you find that on many vacation rental posting websites.
TimeshareMarketplace.com: If you want a time-share in a large resort, this is a good place to start looking.
BidGoTravel.com and BidMyRental.com: These are a bit different. If you like the auction or bidding aspects of, say, Priceline or EBay, these may be for you. These relatively new online agencies use that model, but their inventory isn't terribly robust. With time, though, they could represent a way to save big on accommodations.