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Tag To Do allows you to make multiple to do lists based on varying categories--work, home, grocery list, etc. Within each individual list, you can assign priorities and/or due dates. When an item reaches its due date, the application sends a message that appears in the notification area of the phone to let you know the date has arrived and you haven't checked off the item as done.
This combination of flexibility and notification makes Tag To Do a useful tool for anyone trying to keep themselves organized. Best of all, it's free.