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

Zendesk - Zendesk is a beautiful, lightweight help-desk solution.

Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.

Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community

Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.

HelpScout - Help Scout is a simple, straightforward way to provide excellent support