UPDATED: Now allows you to use loans where you do not deduct interest.

This form allows you to compare what would happen if you took one of two choices with some extra cash you have -- prepaying your mortgage each month, or investing it instead. This tries to take into account your tax situation and assumes you always itemize (even late into your mortgage when your interest will be lower -- I am assuming you give to charities, and pay state and local taxes too!!)