Teem - Cloud-based meeting room booking system
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
AskCody - Meeting management system for Outlook & Office 365.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Skedda - Offices, co-working spaces, universities, sports venues, studios & churches are just some of the places that manage the use of their desks, rooms, courts, studios, halls and all other 'spaces' with Skedda
The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.