RSS Manager is a WordPress plugin by ajayve and sputnik1818 that gives you the ability to change the appearance of your RSS feed.
You can easily add images from custom fields, read more link, category list, tag list and any custom code before or after the feed items. Also, the plugin allows you to delay feed publication for several minutes. It is helpful, when you made some mistake and you will have time to fix it before the post went to RSS and all other services like facebook or twitter.
Download and info: http://wordpress.org/extend/plugins/rss-manager/