Trello is a simple and intuitive web-based project management application that prioritizes lightweight functionality and accessibility over a broad feature set. It organizes projects into Kanban boards that allow you to see, at a glance, whether the various tasks are planned, in progress, or done. Trello is very versatile and can be used for anything from agile software development to personal tasks; however, it does not provide any reporting or analytics, and is not suitable for high-level management.
Trello's free service comes with unlimited boards, lists, and cards, and allows for any number of collaborators. Limited power-ups are also available for extended functionality.