Apps
Let's start this overview of Booking.com website with the small extras that could make a difference for users: Apps and Extensions.
You can find similar functionalities of Booking.com website with Booking.com Apps for iPhone, iPad and Android phones and tablets. Providers can offer mobile discounts, which are visible only to app customers, on top of loyalty program and some other offers. Booking with the app you could eventually save some money.
Fig. 1 - Booking.com on Apple's App Store

Fig. 2 - Booking.com on Google Play

Now, if you think about it, property owners must be spending more time on Booking.com than guests do. Of course there is an App for them too, this is called Pulse, it's designed for property owners and it's also found in Google Play and Apple store.
Fig. 3 - Booking.com App for Property Owners

Extensions
Note: We think extensions in many cases are not such a big deal but, since they exist and you may come across them, it's better to touch the subject.
Browser extensions are small pieces of software which add functionality to your browser. They are browser specific (Chrome, Safari, Firefox, Edge, Samsung Internet, Opera, etc.), so you should search for Booking.com extensions extensions which work with your browser. Note that these are usually third party software.
We've found extensions which add a button to your browser, or to your app list, so you don't have to type booking.com in the address bar. They don't add much functionality to your browser. You can get the same result without installing extensions, just bookmark booking.com or add it to your favorites. We would not waste much time with them.
An extension called No Stress Booking promises to remove the alerts and stressful messages which appear on Booking.com site. We tried the Chrome version, it actually cleared the screen from some annoying Booking.com messages - "book now" and "last room" type of messages.
If interested you can make a google search for: "Booking.com extensions for Chrome" (or replace "Chrome" with your browser), and see if any extension is worth trying for you.
Travel Deals
On Booking.com you can find hotels, Flights, cars and activities. The user interface is quite friendly. Search results, even on maps, can be sorted and filtered in practical ways so that everyone can focus on its individual travel needs and budget.
When you make a search on booking.com, the results will show to you all available rates including any discounted rate, deal or offer. That's the easy and effective way to use Booking.com.
The Deals Page, usually linked from the home page, can provide discount travel ideas, if you are open to suggestions. These deals appear also in the search results, if available for the place and dates of your search.
The Secret Deals promotions could be available, if a provider has loaded such promotions, for people who have a Booking.com account or subscribe to Booking.com newsletter (other users can't see the Secret Deals).
Booking.com's Genius loyalty program, for people who register and qualify, envisages some rewards as discounts and upgrades.
Businesses, from individual to large employers, can register on Booking.com for Business, this should help manage employees travel and control travel costs.
Hotel reviews on Booking.com are quite helpful and are made by guests who have completed the stay at the property (Better than some other sites where anybody can post a review). Nevertheless some judgement is required, for instance consider that: Booking.com can remove reviews which do not meet their standard; A hotel could get false positive reviews; Things change and a property could not be as good or as bad as it looks in the reviews; etc.