Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Teem - Cloud-based meeting room booking system
Geary - Geary is an email application built for GNOME 3.
AgilQuest - AgilQuest is a powerful workspace and meeting room booking software that delivers organizations with a central platform from which to find and reserve conference rooms.
Evolution - Apps/Evolution - GNOME Wiki!
MeetingRoomApp - The best meeting and conference room booking system with a number of unique functions for effective meeting management and optimization of company processes.