The Command Line Interface of the Todo.txt task tracking system allows extension via add-ons (which supply customized or additional actions) and filters (which influence the output of tasks).
This is my personal set of extensions (some taken from other authors, but most implemented by myself) that tailors Todo.txt for my own workflow.
If you find any of these useful, feel free to take them. All code is published under the GPL, like Todo.txt itself. Discussions and questions around the Todo.txt workflow is best directed to the Todo.txt Mailing List.