I have never registered a piece of shareware faster than I did this morning after trying out Sciral Consistency. This is a to-do manager for repetitive tasks that can occur within a range of days (for instance, you need to do the bills twice a month, but an Outlook repetitive task scheduling them for “5 o'clock on the 15th and the 1st” is overly rigid. Or, you want to make sure that you keep in touch with old friends at least once a quarter.)

The interface couldn't be more intuitive: a matrix of days versus tasks, with task status color coded, and a simple dot indicating completion (and recalculating the task going forward from that day).