Manage your to-do lists more effectively with my picks for the top four best to-do list managers on the Web. These lists are all simple to use, completely free, and can help you make your to-do lists more productive.
Remember the Milk is a fantastic online to-do list manager that gives you many options, including the ability to be reminded of your tasks via many different methods. You can read more about Remember the Milk here: Manage Your To-Do List with Remember the Milk
Toodledo is a free online to-do list manager that gives you a ton of organizational options, such as folders, subfolders, due-dates, priorities, tags, contexts, goals, notes, time estimates, and more.
Todoist is an extremely user-friendly to-list manager; you can use it to organize your lists as well as create calendars and sub-projects. It's also fully integrated into Gmail and other online productivity tools.
If you're looking for a robust to-do list management site, Nozbe is right up your alley. You can make lists, prioritize projects and tasks, even work collaboratively.