Giant List of OTAs (400+)

A giant list of over 400 OTAs and other travel intermediaries worldwide separated into groups.

As I started building the list, the need for further classification was clear. What is an online travel agency? Is it just a well known site like Booking.com? Or are group booking sites included? What about metasearch and last minute booking apps? Do airline websites that offer hotel bookings qualify? Or only if they work independently?