Front - multi-channel inbox service for collaborative teams
GrexIt - Manage Tasks, Customer Support and Sales right from your Gmail inbox. Turn your Gmail into a powerful collaboration tool.
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
Mailor - Shared inbox, team portal and smart notifications
Missive - Team communication app now supporting FB, Twitter & SMS
Airmail for iOS - One of the best Mac email clients, now on iPhone