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