Leo Baiano

This plugin includes a button that appears when the user scrolls the scroll bar when you click the button the user is taken to the top of the site.
Visit the <a href="https://wordpress.org/plugins/lb-back-to-top/screenshots/" rel="ugc">Screenshots</a> tab to see an example of the plugin running.
<h4>Credits</h4>
<ul>
<li>JS Script <a href="http://www.developerdrive.com/" rel="nofollow ugc">Jonathan Schnittger</a></li>
</ul>

LB Back To Top

Including button that takes the user to the top of the page.

Vijayakumar Selvaraj

Smooth & simple scroll to top plug & play plugin helps to add “Back to top” in your blog. Activate the plugin & see the result. Nothing to configure. 
Note: This is the initial setup. Adding many features in future releases.
Demo:
1) http://oddanchatram.in/ 
2) http://kkeeranur.com/ 
3) http://vijayakumar.org/

Simple Scroll To Top

This plugin is the most efficient way to add smooth scroll to top option in your wordpress blog.

LB Back To Top vs Simple Scroll To Top