My friend Girish has written an awesome Windows tray application for Voo2do. It sits in the corner of your screen and lets you easily add a task, browse tasks by project, and see tasks that are overdue or due today. It uses the Voo2do API. This program requires Windows and the .NET framework. It is released under the MIT license and the .zip file includes C# source code.
Download directly: voo2dotray.zip
Girish and I are both using this client app, and I will continue hounding him to add features just as he hounded me for voo2do.com.