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.
Sortd - Rated the #1 App for Gmail
hiTask - HiTask is a free shared task list for your team. The place to plan, organize, and stay in sync.
KanbanMail - A Kanban board for your emails.
Mail Pilot - Beautiful task oriented email client for Mac, iPhone, & iPad
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.