HeySpace - HeySpace is the only free task and simple project management software with communication and collaboration feature.
CTO.ai - Build, share & run developer workflows in the CLI + Slack
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.
Trello for Slack - Organize anything with anyone, inside Slack
Jeski - Many projects, multiple boards, one Jeski
Troopr Assistant - The AI assistant for Slack