1. JavaScript

<script type="text/javascript">$(document).ready(function(){
$(window).scroll(function () {if ($(this).scrollTop() > 0) {$('#scroller').fadeIn();} else {$('#scroller').fadeOut();}});
$('#scroller').click(function () {$('body,html').animate({scrollTop: 0}, 400); return false;});
});</script>

2. Кнопка

<div id="scroller" class="b-top" style="display: none;"><span class="b-top-but">наверх</span></div>


3. CSS-стили

.b-top {z-index:2600;position:fixed;left:0;bottom:50px;width:34%;margin-left:50%;opacity: 0.5;filter:alpha(opacity=50);}
.b-top:hover {opacity:1;filter:alpha(opacity=100);cursor:pointer;}
.b-top-but {z-index:2600;position:absolute;display:block;left:56px;bottom:0;margin:0 0 0 100%;padding:32px 12px 4px;
color:white;background:#D8D5C2 url(http://ya-knyazev.ru/images/top.png) no-repeat 50% 11px;border-radius:7px;}