Loan-U-Later is a simple loan/investment calculator that can calculate nearly any aspect of a loan/investment, including the interest rate.

Sidenote: At the point of posting this program has been marked as beeing freely available without costs. As this program is hosted with the AppStore the status might change. Authors like to publish their programs for free for promotion purpose and then make it commercial. Also it could happen that the author decides to take the program down.

http://appshopper.com/finance/loan-u-later