As many know, Spotlight in the menu bar provides calculator functions. If, for example, you enter
100+4
you'll get a listing in the Spotlight results showing
100+4 = 104
As long as this listing is selected, using the keyboard shortcut for Copy, Command+C, will place the result of the equation on the Clipboard.
[crarko adds: I tested this in 10.6.4, and it works as described. I know we've run hints about the Spotlight Calculator before, but I couldn't find this perhaps obvious, but still useful, one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20101021142527546