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The basic Trello service is a free, Web-based project management application that's very visual. Free iOS and Android versions are also available.
Wikipedia's entry reads, "Trello uses a paradigm for managing projects known as kanban, a method that had been popularized by Toyota in the 1980s for supply chain management. Projects are represented by boards, which contain lists (corresponding to task lists). Lists contain cards (corresponding to tasks). Cards are supposed to progress from one list to the next (via drag-and-drop)…mirroring the flow of a feature from idea to implementation. Users can be assigned to cards. Users and boards can be grouped into organizations."
General reviews are quite favorable.