Remember The Milk (iOS, Android; free)
Creating a to-do list is one thing -- actually remembering to do the things on your list is another. Remember The Milk, available on the Web and your smartphone, is designed to make sure you stay on task by offering reminders via push notification, email, instant message, text message and even Twitter, if you choose. The app syncs with your account on the Web, making it easy to access your daily tasks from any platform, and lists can be prioritized and shared with others.
Remember The Milk