I can safely assume that everyone knows the venerable Calculator that ships with Windows. Now there is the download for Microsoft Calculator Plus, a nicely spruced up version (non-classic view):

In addition to the visual enhancements, you can see to the left the addition of mathematical functions (well, in the end I still use my HP 48, but...).

The calculator sports Standard and Scientific view, the new Calculator Plus sports a third one - Conversion:

Now that's really nifty!