If you’ve got a password saved, but can’t remember what it is, you can use this bookmarklet to reveal it.
Drag the image to the bookmark bar:

Click the button above to try it out:

If you’re interested in the JavaScript source:

javascript:Array.prototype.slice.call(document.querySelectorAll("input[type='password']"))
    .map(function(el){el.setAttribute('type','text')})

It gets all password inputs and turns their type to text; and should work in modern browsers (like Safari, FF4, Chrome etc) that support ‘querySelectorAll()’.