Makagiga is a free and open source multi-platform simple software for creating to-do lists, notes, and reading RSS feeds

Purple Task is a free and open source simple software for managing tasks

OpenTodoList is a free, simple, cross-platform, open source software for creating notes and task lists

Planner – free and open source software for tracking your tasks, projects, and goals for Linux

Akiee is a free and open source cross-platform program for maintaining a task list with Markdown support

Kapow – free and open source multi-platform software for time tracking

Task Coach – a free, open source, cross-platform personal task manager designed for keeping track of personal tasks and to-do lists