An interesting productivity tool that I've recently seen discussed online is Sciral Consistency. This application has been around for a while, but an updated version was recently released, and it's this announcement that brought it to my attention.
The basic functionality of this program is to manage "fuzzy" interval tasks that don't have a concrete due date. Examples of this are "check my credit report once a year", "get a haircut every 3-5 weeks", or "water the house plants every 3-5 days". The application lets you set a range of days during which the task should be completed, as opposed to a specific due date. You get a tabular view of your tasks and their corresponding time ranges, and the colour coding indicates how consistently you have been completing each task.
There is a free, unregistered version available at the Sciral website, and certain limits (presumably on the number of tasks you can enter at once, etc.) can be removed by paying $25 to register the product. For those who struggle with staying current with their more loosely-scheduled tasks, this looks like it might be really useful.