To Do Application
Points to Cover:
To Do list can be sorted by:

Priority, Due Date   This is the default sort order.  All priority 1 items are listed first, then priority 2 items, and so forth.  Items within a priority are sorted with the closest deadlines first.

Due Date, Priority   Puts past due items at the top of the list, then the closest deadline items after that.

Category, Priority   Sorts by Category (Business, Personal, …), then by Priority within the Category. If it is visible, the item category can be changed by tapping on it.
Category, Due Date   Past due items are placed at the top of the list, then within category, items are sorted with the closest deadlines first.

NOTE:  The exclamation point means that the To Do item is past due.