My friends John and Heather have gone all Web 2.0 and put qwer.us in beta. It's a free online to-do-list-keeper they made which focuses on "simplicity, extensibility, and robustness". These are probably the three things I crave more than anything else when it comes to software, especially online, so I recommend it wholeheartedly.

(Of course, my personal to-do list is just an index card with "read English book, then turn over" on one side and "read Japanese book, then turn over" on the other.)