Asvin Balloo

This plugin will add a “Back to top” icon automatically to your blog. It features a smooth scrolling effect and customizable options such as the color, position and scroll duration, sprinkled with some cool CSS3 effect.
By <a href="http://htmlblog.net/" rel="nofollow ugc">Asvin Balloo</a>.
Supported features:
<ul>
<li>Smooth scrolling animation</li>
<li>Customizable options via the admin panel</li>
<li>Works on almost all browsers</li>
<li>No need to add any markup to your theme</li>
</ul>

WP To Top

This plugin will add a "back to top/ scroll to top" icon automatically to your blog.

PluginsPoint

Kento Scroll. Jump. Top. Is a simple and very light plugin that will take your visitors to the top of the page once they are scrolling through your page.
Plugin Features
<ul>
<li>Select from 20+ icons</li>
<li>Smooth transition</li>
<li>Easy install and active</li>
</ul>
Plugin Demo and Support: http://kentothemes.com/items/plugins/kento-scroll-jump-top/

WP To Top vs Kento Scroll Jump Top