Windows only: No-nonsense task manager ToDoList is small, lightweight, and portable (so you can run it from your thumb drive), and it offers the feature other task managers frequently and strangely lack: the ability to track multiple levels of nested tasks. Here's a screenshot of the nesting in action.
In addition to nested subtasks, ToDoList sports a large settings pane directly beneath the task list itself. There you can set a large number of attributes on your tasks, including colour coded priority, time started and completed, status, category, if the task is recurring and how often, dependency on other tasks, and even a link to related files.
Just below the primary menu bar you'll find quick filter menus that slice and dice your tasks, sorting by due dates, complete/incomplete, status, priority, category, and who the task is allocated to. If the quick filters won't cut it for the search you're performing there is a robust search query with support for rules-based searches.
While the number of features included in ToDoList is impressive and the task display tree is great for managing large and complex tasks, if the software has an Achilles heel it's the lack of syncing with popular applications like Outlook and mobile platforms. ToDoList does have support for plug-ins and add-ons, some are linked to from the main page. ToDoListPPC is one such program helping Windows Mobile users manage their to-do lists on the go. According to the developer's Frequently Asked Questions, file list syncing between ToDoList and Outlook will be addressed in the next version. ToDoList is a free download for Windows only.