Scrolly
Live example: http://lab.victorcoulon.fr/javascript/scrolly/ (inspired by Nike Better World)
HTML
<div id="item" class="parallax" data-velocity=".8"></div>
You can add data-fit="xx" to adjust your parallax in real conditions.
If it's a simple element, like an image.
$('.parallax').scrolly();
If not:
$('.parallax').scrolly({bgParallax: true});
License
MIT Licence