ROT13 [de|en]code is a nice Dashboard widget that permits the user to encode clear text to ROT13 formatted text, or decode ROT13 formatted text to clear text.
To install the ROT13 [de|en]code widget, when the download is complete, unarchive it and place it in /Library/Widgets/ in your home folder. Show Dashboard, click the Plus sign to display the Widget Bar and click the widget’s icon in the Widget Bar to open it.
To learn more about dealing with the Mac OS X Dashboard you can easily access the Apple support page HERE.