This site compares over 40 different online To-Do lists.
One of the neat things about this site is that it lists the additional features of each program other than the standard options most to-do's have, like creating and deleting things, editing, and adding lists and tasks.
The list includes some of the most popular online to-list sites such as Backpack and Remember The Milk. Also listed are many that look useful but that I haven't heard of.
If the perfect online to-do list hasn't dropped into your lap yet, have a look here.