jquery.isInView.js

  jQuery.fn.isInView = function(){

    // setup different vars for easier access to these values
    var middle = Math.round(this.height() / 2),
        winHeight = $(window).height(),
        posTop = this.position().top,
        scrollTop = $(window).scrollTop(),
        middlePos = middle + posTop;

    if ( middlePos < scrollTop ) {
      // middle of element is above the viewport
      return false;
    } else if ( middlePos > scrollTop && middlePos < scrollTop + winHeight ) {
      // middle of element is within the viewport
      return true;
    } else if ( middlePos > scrollTop && middlePos > scrollTop + winHeight ) {
      // middle of element is below the viewport
      return false;
    }

  }