Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.
Mailvelope - Mailvelope open source project. OpenPGP in your browser.
Missive - Team communication app now supporting FB, Twitter & SMS
ProtonMail Extension - Unofficial webextension for ProtonMail
Front - The platform for exceptional customer service at scale.
GPGMail - GPGMail is an open source plugin for Apple Mail.