Open source: anyone can use, study and modify it, including businesses. Free: no cost, but the code is closed. Freemium: a lasting free plan with paid tiers for extra features. Source available: code is public but the license adds conditions.
FAQ
What is the best free alternative to Trello?
The most popular picks are Wekan, Plane, Kanboard and AppFlowy. Self-hosted and very close to Trello: Wekan. More modern, with sprints and Gantt charts: Plane.