Smart 404
The Smart 404 plugin searches your website for the keywords found in a requested URL that would otherwise return a 404 page not found error and tries to guess which page the user is looking for. If a match is found, the plugin will redirect the visitor to that page. If more than one match is found, the plugin can display a list of suggestions to the user. The Smart 404 plugin is available at http://wordpress.org/extend/plugins/smart-404/.