Simple Loan Calculator - Only three inputs, loan amount, interest and length of loan, calculates the monthly payment for any type of fixed payment, fixed period amortized loan (mortgage, car loan, etc).
JavaScript Mortgage Payment Calculator - Here is a simple JavaScript example for an interactive calculator. It just takes a few inputs and figures out your monthly payment.
What's Missing? Loan Calculator - This simple program finds the missing loan variable (loan amount, monthly payment, interest rate or number of months).