HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
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.
Zoho Connect - Zoho Connect is a team collaboration software that helps teams ideate, discuss, and work together, from any place, on any device.
Trello for Slack - Organize anything with anyone, inside Slack
Agile Tools for Trello - The one power-up all agile teams using Trello need
ProjChat - Manage your Trello workflow right inside Slack