When keeping track of things to do, I prefer paper. But I still find NowDoThis an attractive and useful online tool. It has the virtue of extreme simplicity: a text area of unvarying size in which to enter and arrange an unlimited number of items (a scroll bar appears when needed). After you save a list, your items appear in large bold type, one at a time, waiting for you to click "done." No registration required.
NowDoThis might prove especially useful in managing time spent in front of the computer. I can imagine, for instance, making a list of everyday online reading —
NY Times obitsand so on, so as to limit idle browsing. Putting such a list in the Firefox sidebar looks like an especially nifty way to stay on task. (Please be sure to include Orange Crate Art on said list.)
Arts and Letters Daily
Inside Higher Ed
A timer to limit browsing