Hotels.com is one of those travel booking sites that's been around long enough to have the basics sorted. For Belgium and the Netherlands specifically, they've got everything from Amsterdam canal-side guesthouses to Brussels business hotels, seaside options in Zandvoort, and smaller towns most people don't think to look up. The interface is clean enough, though honestly it looks much the same as every other booking site you've probably used.
The real appeal is their user review system, which tends to be more honest than some competitors. People actually mention things like noisy trams outside your window or lifts that are suspiciously small. You'll also find their price-match guarantee occasionally useful if you spot something cheaper elsewhere, though the terms are a bit fiddly to actually claim. They're owned by Booking Holdings (same parent company as Agoda), so the backend's solid, but don't expect them to be significantly cheaper than booking direct with hotels most of the time.
They run occasional promotional codes, especially around shoulder seasons, though these come and go. If you're flexible on dates and willing to spend ten minutes scrolling, you might find something worthwhile. The app works fine too, which matters if you're booking from a train in Utrecht.