Gmilk is a simple GTK tray application which will periodically check for upcoming tasks, issue a notification, and let you complete the task directly from the notification.
Built on the Adobe AIR platform, YaR Desktop is a full-featured desktop app which runs on Windows, Mac OS X and Linux and includes the ability to track the time you spend on your tasks.
MilkMaid is a Mac OS X app which displays tasks in a floating window (which can be hidden), while providing keyboard shortcuts for quickly adding new tasks.
Tasque is a very cool simple-yet-powerful task management tool for GNOME that integrates with Remember The Milk.
Rainlendar is a highly customizable calendar app, and helps keep your events and tasks in plain view on your desktop at all times.
QuickMilk is a Windows 10 universal app that focuses on speedy operation and support for offline fuctionality, with the ability to sync your changes at a later time.
Install Speak Here Now to start adding tasks by voice from anywhere on your Android device using the built-in “Remember...” command.
Windows Phone apps
Milkbar is a Windows Phone app that includes the ability to add, edit and complete tasks, as well as view tasks in a variety of ways, including map and calendar views.
WinMilk is a full-featured Windows Phone app that helps you manage your tasks on the go. Offline use is supported, and new tasks can be added using Smart Add syntax.
Get My Tasks is a Windows Phone app that allows you to download and view incomplete tasks on your device while you're offline.
QuickMilk is a Windows Phone app that focuses on speedy operation and support for offline fuctionality, with the ability to sync your changes at a later time.
Milkman is a Windows Phone app that includes the ability to set reminders for tasks as well as offering a task counter in the app's live tile.
Plugins and extensions
QuickFox Notes is a Firefox add-on that adds easy note-taking capabilities to your browser. Use the Remember The Milk script to manage your tasks alongside your notes in Firefox.
Milk the Cow is a powerful widget that lets you create, edit and complete tasks from the Mac OS X Dashboard.
For users who prefer to keep their fingers on the keyboard at all times, Find and Run Robot with the Remember The Milk plugin (FarrRmilk) can help you manage many aspects of your task list using only keyboard commands.
StormCows is an extension for Thunderbird/Lightning that allows you to keep calendar events and/or todo items in sync with tasks in your Remember The Milk account.
If you live your life inside Emacs, why not manage your tasks there too? The simple-rtm plugin provides an interface for Remember The Milk within Emacs, allowing you to add and edit tasks.
Moo Later is a helpful Firefox add-on that allows you to quickly save a page as a task in your Remember The Milk account. It automatically uses the title and URL of the current page.
The Vimperator add-on transforms Firefox into a vim-like environment. Add the Remember The Milk plugin to the mix and work with your tasks using keyboard commands within the browser.
Tools and utilities
Take control of your task list via instant message with Jabber2Rtm. With a variety of IM commands, you can manage your task list right from your Jabber client.
RTM Bulk Adder is a tool that lets you add multiple collections of tasks to Remember The Milk using a single web page.
Command line apps
RTM-CLI provides a robust command line client, written in Python. Features include the ability to add and modify tasks, color-coded output, and weekly planner creation.
Milkmaid is a Ruby script that allows you to quickly and easily manage your task list from the command line.
rumember is a Ruby script that focuses on fast and easy task entry from the command line. Smart Add syntax for specifying task properties is supported.