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.
Kissflow - Kissflow is a workflow tool & business process workflow management software to automate your workflow process. Rated #1 cloud workflow software in Google Apps Marketplace.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Pipefy - Pipefy is a process management software that empowers anyone to create and automate efficient workflows on their own without code.
Slack - A messaging app for teams who see through the Earth!
Process Street - Create beautiful rich process documents in a simple to follow checklist format. Fast, free and incredibly simple to use.