The easiest way into project management, and still the best free Kanban board. Trello is cards on a board that you drag from one column to the next, which anyone understands immediately with no training. For a small project, a personal to-do system or a light team workflow, that simplicity is exactly right.
It does not scale to complex, multi-team projects with dependencies and reporting, and heavy use leans on paid power-ups. But for getting organised in minutes without a manual, nothing is friendlier.