I use Thymer for all my planning. The intuitive user interface makes arranging your tasks for today, this week, or someday very easy. It is also very easy to work in teams on tasks or projects. Discussions on tasks can be shared. Adding and changing tasks can be done very fast via text lines, like the QuickAdd feature in Google Calendar. For instance: Call my mom @wednesday @important @every-week @family @action, means that I have a todo to call my mom, every wednesday, that it is marked as important, that it is stored under the family role, and that it has an action tag. Due to this QuickAdd feature it is also possible to twitter and email tasks/todos to yourself using the same syntaxt. For instance the email subject header is the task, and the email body are the nodes that you want to add.
There is also a mobile app that works for iPhone and Android for mobile task adding en editing.

Thymer is my number 2 online tool after gmail.