osTicket - osTicket is a widely-used open source support ticket system.
mTrip - mTrip is a custom mobile application that allows many benefits to tour operators, travel agencies, TMC, OTA, cruises, and different MICE agencies.
Zammad - The Zammad - Help Desk and Ticket System - Community Page
Tour and Activity Booking System - Tour and Activity Booking System is a software designed for in-destination tours, activities, and attractions.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
TBird app - Name your own price on travel