HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
Sortd - Rated the #1 App for Gmail
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.
KanbanMail - A Kanban board for your emails.
Jeski - Many projects, multiple boards, one Jeski
Trello for Slack - Organize anything with anyone, inside Slack