Use an online calculator to do a "time value of money" calculation that shows him how much debt you can run up when you're using a credit card that charges 17% interest.

The NWS Web site features a handy on-line calculator for determining the heat index based on a combination of either temperature and relative humidity or temperature and dew point.

There are various online tools that can help you work out the carbon emissions that you are producing, such as the carbon calculator.