Front - multi-channel inbox service for collaborative teams
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Missive - Team communication app now supporting FB, Twitter & SMS
MailClark - The Slack bot for external communications
HelpScout - Help Scout is a simple, straightforward way to provide excellent support