I was trying to find a simple utility to calculate the amount owed under a progressive income tax system. I couldn't find any so I made this one.
You could also use it anywhere you need a formula, or algorithm to figure out how much of the total to use with each rate. Good examples would be a commission system where the commission rate increases with sales.
WARNING This is for example use only. It may not apply to your local tax laws. I also don't claim this algorithm used is even correct. (But I think it is)