I also recommend iCal. It's not just a useful calendar for scheduling appointments but you can use if for simple reminders too. If you use the alarm feature iCal will have a pop up alert on your computer for when you set the alarm even if iCal is not opened.

If you have an iOS device like an iPhone then it will sync to your iPhone's calendar so that the alarm will also work when you are not at your computer.

I heard those features will be in the next Mac OS upgrade. It will also send you a message that the chore has been done.