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
GrexIt - Manage Tasks, Customer Support and Sales right from your Gmail inbox. Turn your Gmail into a powerful collaboration tool.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Mailor - Shared inbox, team portal and smart notifications
Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.
Airmail for iOS - One of the best Mac email clients, now on iPhone