Other Todo.txt projects implemented in different languages.

Python

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.

A 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

Rust

There's also a Rust version of todo.txt parser. It is also available in Crates.io.

Ruby

• todo-txt-gem

Go

Manage todo.txt using Rofi or Dmenu. Todotxtmenu

Cross platform

Todour is a simple and fast GUI for Mac, Windows and Linux for managing a todo.txt list (written in C++/Qt)

Mac

TodoTxtMac (source) is a minimalist, keyboard-driven to-do manager for Mac OS X that conforms to the todo.txt format spec.

Windows

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)

GNOME 3

A GNOME 3 extension allows you to manage your todo.txt from the titlebar.

• install: https://extensions.gnome.org/extension/570/todotxt/
• source: https://bitbucket.org/bartl/todo.txt-bart.libert.gmail.com

KDE Akonadi

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.

Online / Html

Bulldog is an HTML viewer for todo.txt files: https://github.com/infews/bulldog/downloads

Todo.txt for Chrome is an extension with Dropbox & Google Drive integration, and features such as pending task count, saved filters, and more. Get it here

Mobile

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

Swiftodo, for iOS: Swiftodo in the app store, and http://swiftodoapp.com/

Simpletask, for Android: simpletask-android (Written in Kotlin) .
(Checkout variants for the text files to be saved in Dropbox and to be saved in Nextcloud)

Todo.txt for Android: Simple, light, android app, integrates with Dropbox & Google Drive, includes home screen widgets, notifications, and app shortcuts. Supports Andoird 6.0 Marshmallow and higher. Get it on the Play Store

Thunderbird/Lightning

An extension to integrate a todo.txt file into Mozilla Thunderbird/Lightning task lists is available here (Source code).

Emacs

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

Atom

Syntax highlighting for todo.txt files: https://github.com/todotxt/language-todotxt