Ryan

Adds a section to the bottom of your pages and posts listing any external 
links.
Only links starting with http or https are included so ftp, mailto, etc 
will not be included in the list; although it would be pretty trival to 
edit the plugin to allow them to. Likewise any links that start with your 
blog URL are also excluded.
For more information, you can visit <a href="http://www.nutt.net/tag/link-footnotes/" rel="nofollow ugc">Nutt.net</a>

Link Footnotes

Adds a footnotes section to your posts with any external links

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.

Link Footnotes vs Insert Footnote