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.
Papermind - Papermind Slack App is a collaborative article-editing and document management platform for Slack
Basecamp - A simple and elegant project management system.
Slenke - Project management & team collaboration
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.
Trainual + Slack Integration - Capture knowledge or access training right inside Slack