Via Jenni, Sciral Consistency:
Calendars are great for keeping track of tasks where you need to coordinate with others by setting fixed times and intervals.To-do lists are great for keeping track of tasks that you will do once, and that you need to keep in order by priority.
But there's another class of activities for which neither traditional calendars nor to-do lists are optimal. If you already use a calendar and a to-do list, you're probably trying to wedge these tasks into those tools, without realizing that they really call for a new kind of tool. Sciral Consistency is that tool.
Looks promising—I'll report how I go using it.