Here is some javascript I pick up when I go searching.

javascript:document.body.contentEditable='true'; document.designMode='on'; void 0

This can edit any page. Note: It will not save the page.

javascript: alert("OH NO AN ERROR"); alert("OH NO ANOTHER ONE"); alert("AND ANOTHER"); for (i=1; i<25 ; i++){alert("IT NEVER ENDS!!!")} for (i=xxx; i>8; i=i-1){alert("Only "+i+"more to go.")} for (i=1; i<11 ; i++){alert("Aw man I lost count, let's start from 200")} for (i=xxx; i>0; i=i-1){alert(i)} for (i=x; i<6; i++){alert("NO MORE ERRORS!!!")}

Everytime you see i=x or i=xxx, replace x or xxx with a number. Please try to keep it low though... I wore out my hand trying to click that mouse over 700 times...