It sits in your dock, telling you how many days are left to a certain due date. That due date can be set in its preference window. Thanks to Stéphane Sudre's WBCalendarControl you can enter it nicely. If you want the same in the menu, you can use the Countdown menu extra, which can be found on Britta's excellent super OS X menubar items list. However Days Left only counts full days, and switches at exactly 00:00 hours. (It does so with a timer to not waste usefull CPU cycles, and updates on wake and sleep). I put this little thing right next to iCal and let it show me the Days Left until my next most important day. Use it for whatever purposes you want! It's free... take it!