Andrew Norcross

Runs your RSS feed against the W3 RSS Validator and alerts you of any errors or warnings in a convenient dashboard widget. If either exist, a quick synopsis will explain the issues and provide a link to the W3 site for further explanation and possible fixes.

WP RSS Validator

Runs your RSS feed against the W3 RSS Validator.

Scriptonite

This is a simple yet powerful plugin to repair your wordpress RSS feed. This plugin will help you if you get an error that looks similar to this:
XML Parsing Error: XML or text declaration not at start of entity 
Location: http://www.yoursite.com/feed/ 
Line Number 2, Column 2: 
-^
If you get this error and have not been able to resolve your feed issues than this plugin is what you need. RSS repair adds a new item to your wordpress tools menu. After you do a core update or theme update and your feed stops working, apply the fix and your feed will be as good as new.
Code sources from: http://wejn.org/stuff/wejnswpwhitespacefix.php.html

Fix My Feed RSS Repair

This is a simple yet powerful plugin to repair your wordpress RSS feed. This plugin will help you if you get an error that looks similar to this:

WP RSS Validator vs Fix My Feed RSS Repair