Every time I come across one, it annoys me. I have to copy the text, open a new tab to the ROT13 site, paste the text, and click the translate button.
Compare this to something like a collapsable spoiler button-box where you press a button and it expands and expands a box with the appropriate text underneath it. Even making a [spoiler][/spoiler] tag that gave a black background and equally black text would be better than the current ROT13 solution.
Was there actually a reason for doing things this way? If so, why not just include the ROT13 translation in the javascript that'd open and close the textbox? Comments? Criticism?
Sorry I'm late to the game, but yes, I happen to have made such a website.
http://www.lightning-rose.com/projects/rot13.htm
Text input is converted to raw rot13 text, raw URL, HTML URL, and UBB URL.
Here's an example of a raw URL:
http://www.lightning-rose.com/projects/rot13.htm?Yvtugavat%20Ebfr%20znqr%20guvf%21