Other Todo.txt projects implemented in different languages.
You can do a search for "todo.py" at github.com and there are a number of different flavors:
topydo is an active version with several enhancements and extensions to the original functionality.
more recent and still active version which allows for use with git can be found here: https://git.io/todo.py
This allows you to check into a git repository: https://github.com/lydgate/git-todo-py
This is another version: https://github.com/nuance/smarter-todo
A GUI written in PySide(Qt) https://github.com/mNantern/QTodoTxt
This is my favorite version--it has extensions where you can add due date, links, percent done, etc. https://github.com/abztrakt/ya-todo-py
There is a .NET Windows GUI app that sits in your system tray: https://github.com/benrhughes/todotxt.net/downloads
Windows PowerShell version (think todo.sh for Windows): https://github.com/hartez/todo.txt-PowerShell
Additionally, there was also an AutoHotKey gui version that was advertised a couple months back on the list--maybe it might meet your needs, if you're looking for more of a GUI version... http://tech.groups.yahoo.com/group/todotxt/message/2757 (https://github.com/jdiamond/todo.txt-ahk)
A GNOME 3 extension allows you to manage your todo.txt from the titlebar.
There is an initial Akonadi resource for todo.txt at: https://github.com/markushx/todo.txt-akonadi
TODO.txt todos are listed e.g. in Korganizer.
Bulldog is an HTML viewer for todo.txt files: https://github.com/infews/bulldog/downloads
Todo.txt-Enyo, for WebOS: https://github.com/thrrgilag/Todo.txt-Enyo
Windows Phone 7/8: In the Windows Phone Store and https://github.com/hartez/TodotxtTouch.WindowsPhone
Emacs major mode for editing todo.txt files: https://github.com/avillafiorita/todotxt-mode
Emacs-based application for managing your todo.txt file: https://github.com/rpdillon/todotxt.el