It's just a minor tweak, but the descriptively named Calculator On Top is freeware that delivers.
Our only complaint was that it installed a desktop icon without asking permission, but since Calculator On Top doesn't support hot key combinations, we probably would have added one anyway for quicker access. The program doesn't replace the Windows calculator, but it looks just like it. The only difference is that Calculator On Top does just that--it stays on top of other open windows. Even though it just saves you a click, it's very handy to have when you're working from several different documents at once and need to perform calculations.
Calculator On Top adds only one new function to the basic calculator, but it works exactly as promised and proved to be a handy tool that takes up minimal space.