Anxiety gives you a handy little icon that sits up on the menu bar. Just click the icon and your list pops up in front of whatever you are doing. The list is never bigger than what you have on it, and fades away as soon as you click off of it. The entire list lives in iCal's to-do list simply giving you an easy way to access the list. You can add things from the little window, check things off (they turn green and disappear). Very easy, very invisible. Very free.
Try it out. http://www.anxietyapp.com/