Wunderlist is a great site out of Berlin, which allows you to do exactly that. If you sign up for an account, you can sync you "to-do" list between your phone, computer and iPad. I really like that you can divide your list into categories. For instance, I like to come up with a list to do "Today," "This Week," and "This Month."
Wunderlist gives you an opportunity to "filter" your lists or see them all together. Like many task list sites out there, it allows you to even "star" important tasks and see tasks that you have completed.
Have an iPhone?You can use Siri to dictate your taks and add them to your Wunderlist inbox!
However you use it, it may be a great way to help you stay organized and save those post-it notes for something else!