There's a good one at
www.wildpackets.com/products/
free_utilities
that you might like to take a look at. Alternatively, just google for 'subnet calculator' and you'll find a selection. Also worth bearing in mind that you can do binary conversions using the calculator built into Windows if you switch it into scientific mode. However if you are thinking at all of going for CCNA certification then remember they make you work them out by hand in the exams - they even turn off the built-in calculator in Windows!
