Sitecore Error Manager (Unic)
The main purpose of the Sitecore Error Manager is to show friendly error messages to the user. The user should see an error page, based on a Sitecore item if an error occurres. Initialy it was built to handle 404 errors, but the module now supports all kinds of error responses. Of course responding with the correct http response code and no infamous client redirects.
Please see the Wiki for the complete documentation.