Best to-do app I have used. It offers most of the features that other similar apps also offer, in a very simple user intefface: organize your tasks into lists, use tags, create smart lists, use two-way sync with Google Calendar. Finally, what makes it Excel over the competitors for me is that it also works as a daily/weekly planner, since the calendar view allows to drag and drop pending tasks into the day/week and also schedule start and end times for each task.