Håkan Wennerberg

This is a very simple, yet effective plugin that strips away non standard UTF-8 characters from your various RSS and Atom feeds. A typical symptom that you need this plugin is if WordPress is generating non-valid XML for your feeds.

HW Feed Cleaner

If you ever had trouble with WordPress generating non-valid RSS or Atom feeds, this is the plugin for you.

ktsvetkov

This plugin is designed to “fight” the broken UTF8 
characters on your WordPress powered site.
Install the UTF8 Sanitize WordPress plugin and you 
will be able to fight those ugly characters. Additionally, you can customize 
what actions you want to perform on them – either convert them to their original 
values once and for all by modifing your posts’ database, OR leave all 
your posts intact and instead convert the ugly chars each time a post is loaded 
(all these options are controlled from the WordPress admin panel).

UTF8 Sanitize

This plugin is designed to handle the broken characters that might appear on your blog posts, if their content is converted from/to UTF8 and ISO-8859- …

HW Feed Cleaner vs UTF8 Sanitize