Open Source Mac To-Do Lists Software

Task Coach - Your friendly task manager. Task Coach is a free open source todo manager. It grew out of frustration about other programs not handling composite tasks well. In addition to flexible composite tasks, Task Coach has grown to include prerequisites, prioritizing, effort tracking, category tags, budgets, notes, and many other features. However, users are not forced to use all these features; Task Coach can be as simple or complex as you need it to be. Task Coach is available for Windows, Mac OS X, and GNU/Linux; and there is a companion iOS app.

Collabtive is web based collaboration software. It is an Open Source alternative for proprietary tools like Active Collab or Basecamp. Features include: Projects, Instant Messenger, Tasks, Files, Timetracking, Multilanguage, Basecamp import

ackerTodo is a web-based todo list manager which supports multiple users, recurring tasks, multiple languages, categories and much more! It's written in PHP/MySQL.