Dr. Don's favorite:
My Loan Calculator by CalcNexus.com: This app lets you calculate fixed-rate simple interest payments. It has the ability to load three different loans. As with a standard financial calculator, if you only have three of four variables, it will solve for the fourth (e.g., loan amount, interest rate, term, payment). A nice feature lets you add additional principal payments to see how the loan balance decreases. The application can also be used on the CalcNexus website.