You can use AppleScript as a standard calculator in a pinch -- just type in the equation as you would normally, and leave out the = sign. The answer will be displayed in a window titled 'The Result.' For example, enter (663 * 24) / 2 and The Result will display 7956.0.
Now I know this is useless, but it proved helpful when I found out my beige G3 didn't have a calculator program!
[robg adds: If you're using Apple's Script Editor, just enter the equation as shown, then click Run to see the results at the bottom of the script editing window. And yes, it's mostly useless (but somewhat interesting) :).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040809194351541