This is the most complete to-do list manager available. While it combines features that other popular tasks have invented, it also adds some distinguishing factors that are only available here.
Singularity, to me, is Things on steroids. It is not that clean interface-wise, but it offers start and due dates, organizes repeating tasks very similar to its mentor, is available on many platforms and integrates the calendar like no other task manager out there. Google Calendars can be integrated with bidirectional sync and events can be checked off. This can be a game changer for some out there. Big recommendation!
SingularityApp