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.
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
hiTask - HiTask is a free shared task list for your team. The place to plan, organize, and stay in sync.
GrexIt - Manage Tasks, Customer Support and Sales right from your Gmail inbox. Turn your Gmail into a powerful collaboration tool.
Mail Pilot - Beautiful task oriented email client for Mac, iPhone, & iPad
Front - The platform for exceptional customer service at scale.