I just discovered that Leopard's Spotlight does math. Try it. Bring up spotlight with ⌘+space. Then start typing something like
7pi*8.3 or
sqrt(2)
and the answer is the first search result.
This isn't quite a powerful as the Google calculator. It doesn't appear to do base conversion, and it doesn't let you copy the result for pasting into an application, but it seems handy nonetheless.
