Metastream - Watch streaming media with friends
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Watch2Gether - Watch2Gether is an app that allows multiple people to watch the same content simultaneously and interact with each other while watching.
Geary - Geary is an email application built for GNOME 3.
StreamParty - StreamParty is a watch communiry, that allows you to watch video streams in sync with the people you care about, on multiple services.
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.