Microsoft Calculator Plus
If you were able to take a look at the Microsoft Calculator accessory running on, say, Microsoft Windows 3.1 and then take a look at the Calculator on XP or Vista, the most striking thing would be how little it has changed over the years (over a decade). Microsoft does have a free PowerToy, however, that you might want to use instead of the included Calculator.
Since it requires a Genuine Advantage check before you are allowed to download it, you should fire up Internet Explorer for the task. GA doesn't like Firefox.
Calculator Plus has three views: Standard, Scientific, and Conversion. I have the Scientific calculator showing in the image above. One nice aesthetic touch is the transparency effect seen when focus shifts away from Calculator Plus to some other window.
If you like MS Calculator
If you like MS Calculator Plus, then you'll just LOVE Calculator++ from ILOC.COM. Download it here:
http://www.iloc.com/products/calculator/
Calculator++ does everything that MS Calculator Plus does, just better, and it does a lot more. It's an arbitrary precision calculator, so you can set the precision to as many significant figures as you like. Just be cautioned that some operations, like logarithms can take a very long time when the precision is real high. Its default precision is 128, which is more than most astrophysicists will ever need. Unlike MS's calculator, which put the mathematical part in Basic mode when doing conversions, Calculator++ keeps all the scientific functionality present when expanding it to Conversion mode. Also, Statistics mode is not even a mode. You can do statistics at any time, and you don't even have to pop up a separate window.
Here's something you'll definitely like about Calculator++. It doesn't require Windows Explorer or any kind of privacy violating checks. It's written in pure Java, is freeware, and will run on any system running Java 6 or later.
Calculator++ does a lot more than can be reasonably put in a posting. All it features would fill a book.
Ditto that about
Ditto that about Calculator++. I tried it out, and it rocks! It really does outshine the MS calculator in every way. I'm using it on Windows and Linux, and it works perfectly on both OSes.