Glympse - Glympse is the easy way to safely share your location in realtime. No sign-up needed.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Zenly - Zenly is a GPS tracker to locate your friends in realtime.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
OwnTracks - Cross-platform location sharing
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.