I use this “bookmarklet”:

javascript:inText=window.getSelection()+'';if(inText=='')%7Bvoid(inText=prompt('Phrase...',''))%7D;if(!inText)%7BoutText='No%20text%20selected'%7Delse%7BoutText='';for(i=0;i%3CinText.length;i++)%7Bt=inText.charCodeAt(i);if((t%3E64&&t%3C78)%7C%7C(t%3E96&&t%3C110))%7Bt+=13%7Delse%7Bif((t%3E77&&t%3C91)%7C%7C(t%3E109&&t%3C123))%7Bt-=13%7D%7DoutText+=String.fromCharCode(t)%7D%7Dalert(outText)

[Not written by me; I have no record of where I obtained it.]

Put it in your bookmarks bar in most web browsers, and when you click it it will display the rot13 of the selected text, or prompt you for text if there isn't any selection. In Safari the first entries in the bookmarks bar get shortcuts ⌘1, ⌘2, ..., so it ends up that to rot13 something on a web page I just need to select it and press ⌘3.