Windows tip: Use the built-in calculator for date calculations
When I wrote about the TimeAndDate Birthday Calculator, commenter Kevyn was quick to note that you can do "pretty much the same job" using Windows 7's built-in calculator.
That statement may be a bit of a stretch, since Windows' calculator does not compute differences in hours, minutes or seconds. But still, I was pleasantly surprised to discover that it can quickly and painlessly calculate the differences between two dates, or add or subtract days to a specific date.
To access this functionality, just fire up Calculator and go to View > Date Calculation. You can also hit Ctrl-E to get there with the keyboard.
Another useful feature built into Calculator is unit conversion, accessible via View > Unit Conversion or Ctrl-U. You can convert back and forth any number of volume, energy, mass and other unit types. Currency conversion is not included, of course, since that would require constant exchange rate updates.