Ryanair uses OctopusTravel. I used it to get an excellent rate at a nice 3* hotel in Venice.
Priceline Hong Kong uses the OctopusTravel portal if you want to book a "named" hotel.
TripAdvisor links to OctopusTravel in its hotel listings.
I've seen other companies use it too.
The rates also seem to depend on the site you use to enter OctopusTravel. Sometimes the Ryanair site will have deals that other sites do not have, and so forth.