Dekisugi

This “Insert Footnote” plugin add any text or HTML code at the bottom of all post contents. If you are tired of text-insertion plugins that is just too bulky and complicated, this simple and light plugin is ideal for you. Javascripts such as adsense codes can also be added.
The nice thing about this plugin is you don’t get extra blank lines above and below your custom footnote, therefore looks nicer on your posts. Visit <a href="http://dev.coziplace.com/free-wordpress-plugins/insert-footnote" rel="nofollow ugc">Official</a> page.

Insert Footnote

This plugin add custom texts/HTMLs/scripts below all post contents. This plugin is so light.

maurobringolf

This plugin gives you a shortcode to make numbered references to footnotes, directly from your post content. Footnotes are listed and enumerated automatically at the end of your post 
content, reusing all existing styles of your site. All references become anchor links guiding the user to the footnote section of the post.
We would love to hear and discuss your feedback in a <a href="https://wordpress.org/support/plugin/mnml-footnotes/reviews/" rel="ugc">review</a>. If you want to report a bug or have a feature request, please do so via the <a href="https://github.com/maurobringolf/mnml-footnotes" rel="nofollow ugc">GitHub repository</a>.
<h3>Usage</h3>
To make a new footnote reference in your content you can use the shortcode mnml_footnote in the following way:
This is your regular post content and the shortcode by this plugin looks like this [mnml_footnote] Easy, right? [/mnml_footnote]
Everything inside the shortcode will be moved into a footnote and replaced by a numbered reference link to it. In this example the footnote content would be “Easy, right?”.

Insert Footnote vs MNML Footnotes