Well, first I wrote Conversion of decimal number to other notations, then Conversion from decimal numeral system. Now I present the next step - calculator which converts from one numeral system to another.
To get result, you should enter number, i.e. FF, source numeral system's base, i.e. 16, and target numeral system base, i.e. 10.
For systems with base greater than 10 use english alphabeth letters for denoting digits. Thus, 10 = A, 11 = B, 12 = C, and so on.
UPDATE: For fractional numbers conversion visit Conversion of fractional numbers between numeral systems