This is a UK app only because it can only calculates UK tax rates. I am happy to write versions for other countries but I would need help from an interested user.
This app works out the return from a buy to let property. I am working on being able to save the results and recall them so that you can do a comparison between properties.
It lets you select stamp duty rates first home second home and letting fees.
I think I have the maths right and would like messages if you find it is not accurate.