Windows/Mac/Linux (Adobe Air): Desktop to-do application App For The Milk puts your Remember the Milk tasks into a stand-alone Adobe AIR window, with both offline and online support.
Once you've installed the application and entered your credentials, you'll be prompted to authorize the application with your Remember the Milk account and synchronise your tasks down to the desktop client. As long as you are connected to the internet, the client will operate in "online" mode, adding or updating your to-do items immediately, but when you go offline you'll be able to use the cached set of items and they will synchronise again the next time you are online.
The application is still a little rough around the edges, and not everything is supported—the noticeable lack of smart list support will keep me from using it, but it's a very promising new tool for keeping yourself organised with Remember the Milk. App for the Milk is a free download for all platforms, requires Adobe AIR.