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
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.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Nifty - Manage projects, work, and communications in one place.
MailClark - The Slack bot for external communications
CAMPR - Open source project management tool with 16 modules