Kanboard - Kanboard is a free and open source Kanban project management software.
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.
Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
KanbanFlow - KanbanFlow is a Lean project management tool allowing real-time collaboration between team members. Supports the Pomodoro technique for time tracking.
Kinopio - Thinking canvas for mind mapping, brainstorming, and moodboards
Meistertask - MeisterTask is an intuitive task management and collaboration tool.