<div class="content">
    <div class="main">
        Main Content
    </div>
    <div class="sidebar">
        Sidebar
    </div>
    <div class="footer">
        Footer
    </div>
</div>

#content{
  position:relative; /* required */
  height:2000px;
}
.main{
  margin-left:100px;
  border:1px solid rgb(120,120,120);
  height:1500px;
}
.sidebar{
  position:absolute; /* required */
    top:25px; /* required -- does NOT need to be this value, however. */
    left:5px; /* required -- does NOT need to be this value, however.*/
  border:1px solid rgb(8,8,8);
  background:rgba(70,70,70,.9);
  color:#ecebeb;
  width:93px;
}
.footer{
  border-top:1px solid #ff0000;
  height:498px;
}

$(window).scroll(function(){
    var dist = $(window).scrollTop();
    var sTop = $('.sidebar').position().top;
    var mHeight = $('.main').height();
    var userDist = 100;
    if((sTop > (mHeight - userDist)) && (dist > (mHeight - userDist))){
        //the sidebar is pinned now. it won't scroll further.
    }else if(dist < (mHeight - userDist)){
        $('.sidebar').animate({
            top: dist + $('.sidebar').height()
        }, 0);
    }
});