Its name is App for the milk and you can use it on any OS, just instal Adobe Air and then App for the Milk, that's all.
Its interface is simple and easy to use, it respects all sections and original controls you find in Remember the Milk, so it will remind you the webapp.
Create, modify and delete tasks, manage list tasks, add notes, tags and end dates. It's amazing.
Finally, App for the milk will be running in the system tray, by that way, we will access it quickly just clicking its icon.
It requires Adobe Air