Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
Watch2Gether - Watch2Gether is an app that allows multiple people to watch the same content simultaneously and interact with each other while watching.
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
Metastream - Watch streaming media with friends
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
StreamParty - StreamParty is a watch communiry, that allows you to watch video streams in sync with the people you care about, on multiple services.