Google and Yahoo! both have built-in calculators in their search boxes … you just have to learn the symbols and know how how to organize your parentheses if it’s a complicated equation.
But, here is something quicker if you are using a Google toolbar or Safari: with the Google search box in the upper right of Safari’s window:
Just enter the equation right in the search box in the upper right corner and hit enter. The Google search results show the equation and the answer to the equation.