From Intelephant Software:
Intelephant Amortization Calculator calculates Number of Payments, Interest Rate, Principal, or Payment Amount for a Loan. Three of the four values must be entered. It will also calculate a Balloon Payment. When all values are filled in, you can generate an Amortization Schedule. Intelephant Amortization Calculator will also allow you to enter Extra Principal to be paid each month, and will calculate how much interest and now many payments will be saved as a result. Finally, you have the option of printing your amortization schedule.