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

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.

Link Footnotes vs Annie