React Native Desktop - Build OS X desktop apps using React Native
Roundcube - Web-based IMAP email client
Arbiter IDE - The offline-friendly, in-browser IDE for pure JS prototypes
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
CodeOnline - A remote and secure workspace powered by VSCode
HEY - Email at its best, new from Basecamp.