I recently discoved the Google Calculator, and since then I've not had any need for the Calc application in Windows or OSX.

The thing is that you can google for a mathematical expression, e.g. 3 + 9. More advanced expressions, using functions such as square root, sine etc. is of course also possible.

What's really cool is that the calculator can work with units. For example 50 meters / 3 seconds gives the result in meters per second. 60 watt * 24 hours gives the result in joules.

It's also possible to convert units, e.g. 3 km in miles or more advanced conversion such as 60 watt * 24 hours in calories.

Pretty cool, isn't it?