Check out this great app - its called AirCalc and just like the name suggests, its a floating calculator! You can run the app on top of any other app in a floating fashion but if it blocks your view, you can minimize it to take up less space and then maximize it back to its original size!
Unlike other conventional calculator apps where you need to switch to it, then back to the application that you are using (such as a finance app or online site), this one stays on top of your view so you can easily access it anytime and quicker too. So check it out - its completely FREE!
AirCalc (FREE) - Android Market