Try using the element.getBoundingClientRect() like this:
var rect = element.getBoundingClientRect();
console.log(rect.top, rect.right, rect.bottom, rect.left);
Some browsers also return height and width properties, though this is non-standard. If you're worried about older browser compatibility, check this answer's revisions for an optimized degrading implementation.
Internet Explorer has supported this since as long as you are likely to care about and it was finally standardized in CSSOM Views. All other browsers adopted it a long time ago.
The values will be returned by element.getBoundingClientRect() are relative to the viewport.
But, if you need it relative to another element, simply subtract one rectangle from the other like this:
var bodyRect = document.body.getBoundingClientRect(),
elemRect = element.getBoundingClientRect(),
offset = elemRect.top - bodyRect.top;
alert('Element is ' + offset + ' vertical pixels from <body>');
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