donebox is probably the best to-do list manager ever created. No less.
This application was originally written by Marc-André Cournoyer (the father of Thin: http://code.macournoyer.com/).
I (Thomas Lecavelier) just upgraded it to rails 2.1.0 and plan to fix some strange behavior and add some features.
I plan to implement a full set of features, but only in a plugin way: donebox has to stay as light as it can, so that admins can install only what they want to propose and users can enable only the features they want among the installed plugins.