The Rawlinsons mobile phone app has many useful calculators on it. For example, it can help you to work out how much income tax and national insurance you will be paying, or how much that new company car will cost you on your tax.
There are also useful tax tables, such as the main social security benefits, tax rates and tax allowances. And if you want to save some tax, why not look through the tax savings schemes?
IMPORTANT - the Business Mileage Tool requires the use of GPS. Continued use of GPS in the background can dramatically effect battery life.