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.

fractal512

Fully adjustable scroll to top tool for your site. Now with SVG icons! 
Easily adapts to any theme. You can change type (button or sidebar), size, shape, icon, color and much more. Dynamically binds to the page and, thus, does not affect SEO. This plugin uses Color Picker jQuery plugin for easy way to change colors of Your button or sidebar. Tested in browsers: Internet Explorer 7 – 11, Microsoft Edge, Safari, Google Chrome, Mozilla Firefox and Opera.
Features:
<ul>
<li>type of scroll tool (button or sidebar);</li>
<li>size and position of the tool;</li>
<li>colors of the button/sidebar;</li>
<li>disable for small mobile screens option;</li>
<li>animated scroll motion option;</li>
<li>translucency of the tool option;</li>
<li>scroll down to the last viewed position option.</li>
</ul>

Scroll tool

Fully adjustable scroll tool for your site.

WP To Top vs Scroll tool