This Greasemonkey script encrypts the contents of a text box on a web page using the rot13 cipher at the click of a button.
If there is a clean text box on a page, the e13 button pops up next to it. Click on it and the text inside the box will be rotated by 13.
well, here are some examples of the operation

Get the scripts here
http://userscripts.org/scripts/show/9234

Note:
You will need Firefox and Greasemonkey to be able to run this script.
Get Firefox here
Get Greasemonkey here

Here is a related post which explains how to install Greasemonkey.
Comes with absolutely no warranty.