I’ve been using http://www.rememberthemilk.com. Like most todo lists (even paper) you have to remember to look at them, but once you do, they work well.
Remember the Milk works better than any I’ve used on paper or computer. The ability to subscribe to your todo lists via RSS is pretty cool, and you can send todo items to other users or just plain share them with other users. I just don’t know anyone else using the service.
The UI is absolutely beautiful. AFAICT it is all AJAX, which is good for the most part, unless I want to use my browsers back button. The beautiful UI is FAST. I guess that speed is what comes with everything running on the client and all server calls running asyncronously in the background. Not having to wait for the server is pretty cool.