On Windows, use Alt-PrtScr to take a screenshot; you can then paste it into an e-mail message.
On Mac OS X, use Command-Shift-3 to take a screenshot (or if you only want to send a single window, use Command-Shift-4, position the mouse over the window you want, and then use the space bar). This will create a file on your desktop that you can drag into any document, such as an e-mail message.

(instructions courtesy Jerry Stratton)