I have a common problem - limited ways to see how to pay off my mortgage. Spreadsheets are great at seeing a single payment, but seeing the impact of larger payments to the principal are really helpful for budgeting. I want to create a website calculator that provides this information in a simple way, and people can download/get emailed their custom payback plan. I have the basic functionality modeled in a spreadsheet, but it obviously needs to be ported to a real website/landing page.
Ideally, this would have reasonable SEO so that I can leverage posting ads to help defray hosting costs.