Fire up Safari and type an expression into the Google search field. Hit enter and Google gives you the result. As well as simple arithmetic (eg 5+2*2), it can also do more complex stuff like sine(30 degrees) or e^(i pi)+1. It knows about physical constants and can also handle conversions for example, 100 miles in kilometers or 160 pounds * 4000 feet in Calories.
[robg adds: Obviously, you can do this from any application that accesses Google; it works really well with the pop-up search engine front-ends such as Searchling and Huevos.]

