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.
Kanboard - Kanboard is a free and open source Kanban project management software.
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.
Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Kinopio - Thinking canvas for mind mapping, brainstorming, and moodboards