There's a Firefox extension called "leetkey" with a metric fuckton of features, which I have to use only the feature that lets me translate ROT-13. It's still pretty irksome. (The metric fuckton of features means I have to do more menu navigation than would be most convenient.)
It's the one I use, too. But you don't need to navigate menus: if you open the settings, you can bind the ROT13 switch to a keypress.
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?