Extending jQuery with an "inview" selector to select elements that are in the visible part of the page (at least partially).

jQuery "inview" selector

$.extend($.expr[':'], {
    inview: function ( el ) {
        var $e = $( el ),
            $w = $( window ),
            wt = $w.scrollTop(),
            wb = wt + $w.height(),
            et = $e.offset().top,
            eb = et + $e.height();

        return eb >= wt && et <= wb;
    }
});