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
Azendoo - Azendoo is a collaborative task management solution that helps companies to get work done easily and in teams.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Taskade - Make lists, organize your thoughts, and be inspired to get things done. Taskade is a collaborative space for your tasks.
MailClark - The Slack bot for external communications
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.