Desktop apps
Gmilk
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.
Rainlendar
Rainlendar is a highly customizable calendar app, and helps keep your events and tasks in plain view on your desktop at all times.
Tasque
Tasque is a very cool simple-yet-powerful task management tool for GNOME that integrates with Remember The Milk.
Plugins and extensions
FARR Remember The Milk Plugin (FarrRmilk)
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.
SimpleRTM (Emacs mode)
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
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.
Tools and utilities
Jabber2Rtm
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
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
rumember
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.
RTM-CLI
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
Milkmaid is a Ruby script that allows you to quickly and easily manage your task list from the command line.