There are about 50 or 60 zillion online "calculators" floating around out there on the "Internets" so how do you pick just a few that are useful for your students? After all, we have calculators that will tell you that eating seven Baconators is a bad idea. Calculators that can tell you how many days until the yearly Little 5 Points Halloween parade. Heck, there is even this.
All of those are great but I'm thinking these three would be quite helpful during that personal finance unit you keep putting off until later. You've got your savings calculator, your budget planner calculator and your credit card pay-off calculator.