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.

Chris Roberts

Annie provides comprehensive annotation options, including footnotes and highlighting. If Tippy is installed, footnotes can be displayed inline using tooltips. Also, footnotes are placed at the end of the post. For highlighting, you are able to use default highlighting styles or define your own.

Annie

Provides comprehensive annotation tools for WordPress posts.

Insert Footnote vs Annie