Not sure exactly which effect it is you are referring to but presumably it is created somewhere in the following JavaScript which that page has attached to it:
Code:
<script id="LR1" type="text/javascript" src="http://platform.twitter.com/js/tfw/hub/client.js"></script>
<script type="text/javascript" src="http://www.google-analytics.com/ga.js"></script>
<script src="/javascripts/mootools-core-1.3.2.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/mootools-more-1.3.2.1.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/MediaViewer.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/css_browser_selector.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/powertools-1.1.0.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/navigableFilmstrip.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/homeCarousel.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/AlphabetGallery.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/handlebars-1.min.js?1342226051" type="text/javascript"></script>
<script src="/javascripts/jquery.min.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/jquery-ui.min.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/rails.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/application.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/jquery.fancybox-1.3.4.pack.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/jquery.easing-1.3.pack.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/jquery.mousewheel-3.0.4.pack.js?1340892453" type="text/javascript"></script>
<script src="/javascripts/jquery.qtip.min.js?1340892453" type="text/javascript"></script>
<script src="http://platform.twitter.com/widgets.js" type="text/javascript"></script>
<script type="text/javascript" src="http://use.typekit.com/jlb0uph.js"></script>